Unfortunately, we find that the Skype for Business IM window freezes when you want to write a message when Competella is installed on your computer.
The solution is first and foremost to install the latest version of Competella.
You can find it right here.
Once you've done that, open Settings in Competella.
Here you must uncheck the Hide Skype conversation window under Skype in the Call handling area. Click OK to confirm the change.
This solves the problem.