Thanks for your reply.
I have updated it to 2.5.28
Still having the same issue.
I am using Easy Calc Check Plus
Disabling the plugin also does not make a difference.
extensions.joomla.org/extension/access-a-security/site-security/easycalccheck-plus
Update:
I have also updated CB to 2.05 but it gave me this installation error:
1064 - An error has occurred. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 3 SQL=UPDATE el5sc_menu SET lft = lft + 2 WHERE lft >
The site is now broken, so after a reinstall of CB 2.05 it gave no installation errors, but the site is still broken.
Update 2
If I disable the Community Builder Login module the site "works" again, but without this module functionality.