website down and malfunction after disk full

Hi,
It appears that our error log kept growing and the server went down. We then removed old backup data, and deleted the /var/www/vhosts/petelements.hk/PE/system/logs/error.txt file and recreate a new one. We tried rebooting the server but no use. We noted that the admin login screen can be shown, but after typing in username and password, the browser went to an url like http://www.petelements.hk/admin/index.p ... 4ec00ba550 but the screen is blank. I know the opencart we use has a server cache installed. There are many files in the /var/lib/php/session folder. What do you think we can do to recover the website? Any tips appreciated, thank you.

Contents of error_log:
[Sat Feb 17 09:38:24 2018] [error] [client 220.181.108.183] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 09:54:30 2018] [error] [client 123.125.71.76] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 10:24:34 2018] [error] [client 180.76.15.28] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 10:58:22 2018] [error] [client 14.215.176.14] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 15:56:16 2018] [error] [client 207.46.13.46] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 19:34:42 2018] [error] [client 46.229.168.70] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 19:47:06 2018] [error] [client 62.210.101.101] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Sat Feb 17 21:10:28 2018] [error] [client 46.229.168.71] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Tue Feb 20 08:03:33 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 08:03:33 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 09:25:16 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 10:34:37 2018] [error] [client 192.241.219.185] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Tue Feb 20 11:16:55 2018] [error] [client 180.76.15.26] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Tue Feb 20 11:20:24 2018] [error] [client 221.124.95.219] Directory index forbidden by Options directive: /var/www/vhosts/petelements.hk/PE/image/data/Charity/
[Tue Feb 20 12:18:06 2018] [error] [client 157.55.39.208] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49
[Tue Feb 20 16:24:29 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 16:24:29 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 16:40:37 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 16:40:37 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 16:44:04 2018] [error] [client 219.78.149.31] user annie not found: /plesk-stat/ftpstat
[Tue Feb 20 16:53:59 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 16:53:59 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 17:07:21 2018] [error] [client 58.152.207.149] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49, referer: https://www.google.com.hk/
[Tue Feb 20 18:35:43 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 18:35:44 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 18:38:55 2018] [error] [client 59.148.57.158] PHP Notice: session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/session) failed: Permission denied (13) in /var/www/vhosts/petelements.hk/PE/system/library/pagecache.php on line 49, referer: https://www.google.com.hk/
[Tue Feb 20 18:57:10 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 18:57:10 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 19:02:05 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?
[Tue Feb 20 19:02:05 2018] [warn] RSA server certificate CommonName (CN) `Parallels Panel' does NOT match server name!?

Well, in a professional Setup, there should be no Error Logs. And you did
not check your Installation, and clear out any Cache and Error Sections for
a long time, as it looks. But first DISABLE this PageCache Function, and clean
out the Page-Cache File Directory, since this function does not work, exept
for producing endless Error Entries.
Ernie
---
Cache Cleaner Tools for Version 1.5.6.x:
https://www.opencart.com/index.php?rout ... _license=0

We know the Cache module can cause the whole website not displaying properly. But when we dig into the log messages, we noted that there is a oc_customer_online table that is reported to be corrupt. We tried to export the table, delete it, and re import it - only to see error saying that a key already exists.
If the corrupt DB table is causing the website to fail presenting, what can we do? The more problems the site has now, the more we feel wanting to hire an opencart expert here to rebuild the online shop. What kind of spending is required for a job like this?
In the past when I took over the site after previous staff left, I noted that the cache module can cause the website not displaying or malfunction. However there seems a bug so that in the admin screen i cannot turn off this feature. So it is a pain in the ass for me.

Sure, you can turn off the Page Cache Extension, at least, if you have this
Extension installed. And you can remove it as well, by comparing the Shop
Site Files with the Content of the downloaded Extension, and remove, what
belongs to the Page Cache Extension. (AFTER you switched it off in the Admin!)
---
Free Page Cache for OC 1.5.X
https://www.opencart.com/index.php?rout ... n_id=19023

---
and you should install something like this, in order to clean out your Cache Sections regularely:
---
Clear All Caches
https://www.opencart.com/index.php?rout ... n_id=18823
---
cache remove
https://www.opencart.com/index.php?rout ... n_id=23294
---
Ernie
---

It seems that your session folder is full.
So session files can not be writen anymore, wich in term is causing your admin login problems.
Session folder is located /var/lib/php/session

Clean out that folder.