Does anyone have any Idea how to solve this error? I need to fix this as soon as possible.

Hi Christian,

Good news: I've identified the problem.

Your ost_user_account table is missing the timezone column, which was introduced in a recent osTicket core update. It appears the database migration during your upgrade from 1.18.1 to 1.18.3 didn't complete successfully.

Run this in your MySQL console:

ALTER TABLE ost_user_account ADD COLUMN \timezone\varchar(64) DEFAULT NULL AFTER \status\;

Now, I have to be transparent with you. When you confirmed that the same error occurs in old osTicket mode (SHIFT+O), that tells us this is an upstream osTicket core issue, not an osTicket Awesome issue. Normally I would direct you to the osTicket community forum at that point, because I simply cannot troubleshoot core osTicket problems for all of my customers. If I did, I wouldn't have any time left to actually develop osTicket Awesome!

I overlooked your answer to my fifth question above, so I kept digging when I should have pointed you upstream. Since I've already found the answer, it would be silly not to share it. But please understand this is a one-time courtesy, not a service that comes with osTicket Awesome. I also assume no liability if something goes wrong with the above query. Please back up your database before running it.

For future reference, any issue that also occurs in old osTicket mode (SHIFT+O) is by definition an upstream issue. The best place for those is the osTicket community forum:
https://forum.osticket.com/

Hope this gets you sorted!