PHP Fatal errors

Hello,

I’m evaluating security plugins for my word press sites. I really like ithemes security but it produces fatal PHP errors.
When iThemes Security is activated I get the following error messages:

[28-Nov-2016 08:22:41 UTC] PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /www/htdocs/rsieber/blog/wp-includes/wp-db.php on line 2406
[28-Nov-2016 08:26:10 UTC] PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /www/htdocs/rsieber/blog/wp-includes/wp-db.php on line 2406
[28-Nov-2016 08:29:54 UTC] PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /www/htdocs/rsieber/blog/wp-includes/wp-db.php on line 2406
[28-Nov-2016 08:33:11 UTC] PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /www/htdocs/rsieber/blog/wp-includes/wp-db.php on line 2406
[28-Nov-2016 09:33:14 Europe/Berlin] PHP Fatal error:  Cannot redeclare wp_fix_server_vars() (previously declared in 

‹­:58) in /www/htdocs/rsieber/blog/wp-includes/load.php on line 109
[28-Nov-2016 08:36:26 UTC] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /www/htdocs/rsieber/blog/wp-includes/wp-db.php on line 2406
[28-Nov-2016 08:36:37 UTC] PHP Fatal error: Cannot redeclare translate() (previously declared in ˆÿ„:7) in /www/htdocs/rsieber/blog/wp-includes/l10n.php on line 106`

These led to HTTP 500 error messages

I don’t have any error messages, when the plugin is deactivated.

What’s going on?

Robert