Only for completion and may be further investigation or to save some people’s time:
I installed a completly new server VM with the same setup. After finished this, I tested all features and everything worked well. Now I restored zammad with it’s own internal backup and restore scripts.
The new setup began to missbehave the same way. So I decided that the error have to be in the internal settings or the db. Finally, after disabling the ‘time tracking’, everything began to work again.
I can’t reproduce the error (it’s now working with time tracking enabled too) and I have absolutly no clue how exactly the error raises. There was absolutly no visible error message other then the JS error.
May be, the info will help and save someone’s time.