I solved the problem:
Apparently themler doesn't work with the global setting on System "advanced cashing".
It was turned on by mistake (I don't even know, what the difference is).
For anyone out there with the problem, that the modules are just shown on the browser you opened the themler editor:
Go to: Global - System and set the cashing to normal.
Maybe the themler-crew will fix this problem in the future.
I solved the problem:
Apparently themler doesn't work with the global setting on System "advanced cashing".
It was turned on by mistake (I don't even know, what the difference is).
For anyone out there with the problem, that the modules are just shown on the browser you opened the themler editor:
Go to: Global - System and set the cashing to normal.
Maybe the themler-crew will fix this problem in the future.