Cannot create new content 3.1.4
From Joomla! Documentation
Some users report that they receive the following error when attempting to create new content (of any type).
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.
or with maximum reporting on
Fatal error: Invalid opcode 153/1/8. in /var/.../libraries/joomla/table/observer/tags.php on line 159
What is the cause
This appears to be caused by a bug in eAccelerator.
How to fix
Either update to the current Joomla! version or change your caching method to a different option.