I've just started having this same issue, having used boxcryptor without any problems for quite a long time. The advanced section of the error message gives the following:
Issuer: CN=CovenantEyesProxy (5481), C=US
I've been using BC and Covenant Eyes for many months without problems. CE has not released an update since February, so I don't see why that should have changed either.
Thanks for any ideas,