Hi Brostin,
When you are trying to open office documents from within Boxcryptor for Android, the app directly forwards the file to any installed Office application.
In your case you might have pre-installed Microsoft Office but no valid license for it.
You might want to look for free alternatives on the marked, Boxcryptor should then provide you with a "app selection screen" upon opening office files.
I hope this information helps!
Best regards,
Florian