Sorry for your inconvenience!
With Boxcryptor 2.19, we added a mechanism to prevent closing Boxcryptor as long as other apps may need to save their data beforehand.
Boxcryptor waits for certain programs to be completely closed before being able to get closed by the Windows shutdown procedure.
You can alter this list of programs via the "Boxcryptor.exe.config" file in your Boxcryptor installation folder (admin rights required so better copy it somewhere, edit the copy and then override the original):
Find the key "ProcessNamesPreventingShutdownOnSessionEnd" and edit the corresponding value field according to the comment in the file.
We are aware of Outlook not exiting completely upon Windows shutdown on some computer configurations, so Boxcryptor would block the shutdown procedure "endlessly" by waiting for Outlook to close.
We will address this issue within the next update, in the meantime deleting "outlook" from the list should fix your problem.