Joomla + K2 white screen - what is the reason?

Question

Joomla

Goodnight.
Joomla! 2.5.24
K2 v2.6.8 Enable
error output.
We go into the material, there we see the following errors:

Strict Standards: Only variables should be assigned by reference in /home/user/site/plugins/content/jw_sigpro/jw_sigpro.php on line 43
Strict Standards: Only variables should be assigned by reference in /home/user/site/www/plugins/content/jw_sigpro/jw_sigpro.php on line 44
Strict Standards: Declaration of JParameter::loadSetupFile() should be compatible with that of JRegistry::loadSetupFile() in /home/user/site/www/libraries/joomla/html/parameter.php on line 512
Strict Standards: Only variables should be assigned by reference in /home/user/site/www/plugins/k2/k2additonalcategories/k2additonalcategories.php on line 249

When saving the material, we see just a white sheet. At the same time:
a) title, content are saved
b) photos, videos - no
If you turn on the debug mode when opening any material, we will get a 500 error:

JDatabaseMySQL::query: 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '_ additional _ categories WHERE 1' at line 1 SQL=SELECT catid FROM l2poc_ k2 _ additional _ categories WHERE 1 –

An error in the k2_additional _ categories plugin, disabled it - it does not help, the white screen remains.
What did the host say?
Based on the system message of the installed CMS on your site, the problem lies in the database schema version:
The database schema (2.5.24) does not match the CMS version (2.5.19).
How did this error come about?
Here is what the site administrator wrote:

Yesterday I noticed a message that the database schema version and the CMS version do not match. Clicked "Fix". There was a message:
The structure of the database tables is up to date.
Other information
Database schema version (from #__schemas): 2.5.19.
Update version (from #__extensions): 2.5.19.
DB driver: mysql.
66 database changes were successfully verified.
40 database changes did not change the table structure and were skipped.

Many thanks for the help and advice.
UPD
Updated php to 5.5
I managed to catch an error:

Fatal error: Allowed memory size of 10485760 bytes exhausted (tried to allocate 18939 bytes) in /home/user/site/www/libraries/joomla/language/language.php on line 845