Hello.
Please run this command here via Terminal (one line):
sudo kextutil -v 6 /Applications/Boxcryptor.app/Contents/Frameworks/BoxcryptorFoundation.framework/Versions/A/Filesystems/bcfs.fs/Contents/Resources/mount_bcfs.app/Contents/Extensions/10.14/bcfs.kext; sudo ls -laeR /Library/StagedExtensions; sudo kextcache --clear-staging; sudo ls -laeR /Library/StagedExtensions
After executing the command, please restart your Mac and try starting Boxcryptor again and see if your problem has been resolved.
Best,
Yasmin