Image upload doesn't work

Long story short, when I want to add an image using file manager, I get an alert saying "Warning: File could not be uploaded for an unknown reason!". Some images get uploaded, but does not get resized, so image is there but nowhere else can see the thumbnail. If I upload images via FTP, there are no resized versions so, not a solution. Sometimes I got errors aboud decoding, didn't know what was that about. Error log at /system/storage/logs/error.log is empty.

What I tried:
1. Disable gzip compression using .htaccess, php.ini, opencart admin panel
2. changing file name to contain only english letters
3. lowercase file extensions.
4. different browsers, different IPs.
5. changing chmod 777 for images and temp folders.
6. changing owner of those folders to root, apache.

So what am I missing? what other information I should provide?

Long story short, too short!

Sorry, 2.3.0.2

Later on, checked php version, it was 5.5.?? (don't remember, but opencart requirement is 5.4+ so it should have worked) and I upgraded to 5.6.29. It didn't work, restarted httpd service, restarted whole server couple of times. still did not work. But the next day, it work magically.

So for those who got this problem, upgrade your php to the latest, restart your server and wait(???) 24 hours. worked for me at least

Right..! most time this error belongs to php.ini file. Sometime link to image directory permission level.
Here is the solution https://doubtsolver.com/ask/332/opencar ... own-reason