Flapping IP link between two nodes

I’m using ZeroTier to connect from a Mac M.2 to a Ubuntu 20.04 computer. Everything was working fine until a few days. Since two days the connection goes up and down every few minutes (constant flapping behavior), which is making it hard to work with. We can confirm this behavior by pinging from Mac to Ubuntu and vice versa with their respective ZeroTier IP: both ends are losing their ZT link at the same time (only towards each other) while maintaining their connection to the internet.
After taking a few tcpdumps, I noticed that the ping issue occurs when the ZT server destination address switches (UDP packets address). This address seems to be toggling between the two following IP: UDP ports 55671 → 9993 (ZT link working) UDP ports 9993 → 9993 (ZT link not working)
Maybe there is some issue with the 2nd ZT server (
Please let me know how I can troubleshoot this further.

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