If I create a new ticket and add a new Customer there, The Form “New: Customer” doesnt seem to do anything when I click on “apply”. If I wait a little and cancel, sometimes the customer was added and can be selected.
Sounds like an issue with your database server.
If you’re using postgreSQL have a look at your configuration and look for max_connections.
My guess is that it’s set to 100 (which is default) - you may want to raise it to at least 200 or even better 500 or 1000.
I also guess that this issue only occurs if your Zammad runs for a longer time but not after restarting the service. Your logfile might contain sweet hints like Deadlock, ERROR or connections.
Anyway, you should be able to find your psql configuration file somewhere within: /etc/postgresql/*.*/main/
You’re looking for the file postgresql.conf and the setting max_connections.
Restarting postgresql and Zammad (in that exact order) is necessary after adjusting your configuration.