Error when trying to add new page

[reneh]

Not sure here...
you may have a buggy version of mySql - maybe v5.1.something
(Probably before 5.1.47 - as I see a such bug fixed around mySql 5.1.47)

You could also manual look at your db table cms_content and see if you can find some bad rows in there...
And maybe look in cms_content_seq if it report next seq number... and not 0...

And in end also maybe try a repair routine on db - in case something is gone wrong in db? (remember backups!)

Good luck!