Sync: not working: Mac and iPhone: with Proton VPN

Unless your school, employer or financial institute requires the use of a VPN, it's probably best not to use one.

VPNs are not as secure as they are marketed to be and they often cause the problems that you are experiencing.

Network connectivity issues may include data syncing. Please see:

If your device has network connectivity issues, check for VPN and other third-party security software - Apple Support

You're welcome.

You should do a little more research on VPN use.

Apple has built anti-malware protection into the macOS, and they've secured the hardware in ways that make it very secure against malware and unauthorized modification; sealed and signed system volumes, secured biometric keys, etc.

If you're interested, here is some guidance from Apple and the Community re: Apple security.

Effective defenses against malware and ot… - Apple Community

macOS - Security - Apple

Apple Platform Security - Apple Support