I've solved the problem by doing a complete, clean reinstall of your script - including the database structure.
When I first encountered this problem, I did a complete reinstall using your "auto installer", BUT I kept using the existing database. The problem persisted... This obviously points to a problem with the database.
Here is my opinion on what caused this... A few months ago, I moved that site to a new server using the Plesk Migration tool. I've noticed problems with databases after migration using Plesk. I've been doing dumps and recreating databases manually on the new server to resolve this. It's probable I skipped this step with this web site's database... And since the client has rarely used the site, or added many events, the problem did not become evident until an unrelated Zend issue caused the client to notice the calendar was not working...
I will open a support ticket and send you the bad database - in case it will help someone else in the future, but my problem is solved and does not require additional support.
Thank you for your prompt responses...