We've just launched our EE2 based website and are now experiencing an odd problem:
In the control-panel it is no longer possible to save any changes to the sites content entries (or create new ones). When pressing 'Submit' or 'Save Revision' the edit-form is displayed again (containing the changed content).
Everything else on the site and control-panel work as expected, including changing settings and templates.
I activated all possible debug mechanisms (EE-Log, Debug Preference:1 (I am Super Admin), apache-log, Output-Profiler & Template-Debugging), but nothing shows up.
The SQL-Trace doesn't even contain an update statement for the channel tables and I manually tested, that the application user is permitted to access/update those tables.
I've also tried changing the "Process form data in Secure Mode" and "Deny Duplicate Data" optional without noticeable effect.
Any ideas where to look for the problem or any other options to try?