Your link https://www.boxcryptor.com/en/help/faq-and-troubleshooting/macos/#i-cannot-connect says:
I Cannot Connect to the Boxcryptor Servers
Depending on your system or network configuration, Boxcryptor may not always be able to communicate with our servers. However, there are some workarounds for the following scenarios.
Warning: This is no Secure Connection
If you are in an environment that performs traffic inspection, you might not be able to connect to our servers. Examples, where traffic inspection might interfere with Boxcryptor:
-Anti-virus solutions that protect internet traffic
-Company proxy servers
Traffic inspection, techically speaking, is a man-in-the-middle attack. Therefore, it is important to make sure your system or internet connection is not compromised. You can check the certificate information provided, by clicking advanced in the error message.
Mac-Firewall is off, no hotspots, no proxy servers
Any other idea?