Thanks for the reply!
No, I’m not running any VPN.
However I think i found the issue, quick summary for others that might see this behavior:
During testing I noticed it was not possible anymore to ping any server on the internet.
My device is configured as bridge but I added the IP’s on the local network as managed IP’s in the zerotier portal. The default gateway of the local network was amongst them and that seemed to be a very bad idea…
I have now removed all managed IP’s from the bridge and it’s been running like charm since.
It’s kind of a miracle that zerotier was still able to maintain even some remote connectivity