Cannot see other PC connected see my two other PC connected via ZeroTier

I already wrote once about that. I have three PC (P1…P2) which are in a virtual ZeroTier network. I have synchronized certain directories using SyncThing sharing data between all PCs.
P2 and P3 can see P1 in the Win10 network window, but P1 cannot see any one, although the synchronizing works well. I use ZeroTier 1.6.4 and 1.6.5.
Shall I reinstall ZeroTier on P1? Shall I upgrade to version 1.8?
What blocks the few in P1?
In win defender ZeroTier is allowed to communicate
Thanks for further hints. rfa