The aplication is configured to accept page creation / editing only from authenticated users.
When the page was changed and the save action is required, error occurs in function "MediaWikiBagOStuff::_doinsert". The error message is:
A database query syntax error has occurred. This may indicate a bug in the software. The last attempted database query was:
(SQL query hidden)
from within function "MediaWikiBagOStuff::_doinsert". MySQL returned error "1062: Duplicate entry 'wikidb:pcache:idhash:1-0!1!0!!en!2!edit=0' for key 1 (localhost)".
Then the error occurs all the time, even the page was request for first time or by already authenticated user.
After the time stamp from field 'exptime' in table 'objectcache' associated with previous key, the page can be read again, but without the changes.
Well, try w/ 1.9.6 and/or 1.10.0.
As said above, try w/ something newer. This version was removed.