Google drive "online" files cannot be encrypted with Boxcryptor. This is due to the nature of these files, they are actually no physical files which is a requirement for Boxcryptor to be able to handle them. Usually you will see an error message when trying to encrypt a "online" file, I just confirmed that this message doesn't pop up with "*.gslides" files. This will be updated in one of the next Boxcryptor releases.
In the meantime, you might just decrypt the affected slide file, this should restore the link in your browser view.