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I were trying to upgrade my website to version 3.0, After i tried upgrade this happens:
Only a blank page appears with message:
"Error: A theme has not been assigned to this store!", i read about permissions and tried everything a nothing works.

I get back with my backed up files to version 1.5.6.1 and now i got this error messages:

Notice: unserialize(): Error at offset 0 of 5 bytes in /home/emorelos/public_html/index.php on line 69
Notice: unserialize(): Error at offset 0 of 5 bytes in /home/emorelos/public_html/index.php on line 69
Notice: unserialize(): Error at offset 0 of 73 bytes in /home/emorelos/public_html/index.php on line 69
Notice: unserialize(): Error at offset 0 of 17 bytes in /home/emorelos/public_html/index.php on line 69
Notice: unserialize(): Error at offset 0 of 181 bytes in /home/emorelos/public_html/index.php on line 69
Notice: unserialize(): Error at offset 0 of 5 bytes in /home/emorelos/public_html/index.php on line 69
Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/emorelos/public_html/index.php:69) in /home/emorelos/public_html/system/library/session.php on line 11

I tried to solve by remove "SUPER LONG PERMISSION" on user_groups table.
i tried to "settings" table and set unserialized to 0

now i just dunno what to do :( i Cant even log in admin side
Please help my web is www.electronicamorelos.com

Serialize and unserialize functions have been deprecated by the PHP industry. It would be recommended to upgrade to a higher version of Opencart if this issue still persists.