Hello @ntozier . Sorry about that. Completly forgot those things.
osTicket Version: 1.12
MySQL: 5.5.45
PHP: 5.6.40
Can't really give any additional information about what I did. I created a task just a few hours before I opened the ticket and everything was OK. Then somebody else tried it and it didn't work and the things I wrote in my first message happened. I changed nothing between this time. The only thing would be that I restarted the server. Maybe this damaged the database?