Update:
After the above dced, I restarted only the DM PC - again, lost connecting approx 20-21min with same error message.
After that, started DM PC, logged into it with 2nd PC - stable for 3.5 hours and still running. This time the 2nd PC dced (~2 hrs in) but could reconnect to host.
(all scenarios fully afk, no interaction with the program)
Update:
Another day, another testing run.
Test 1 - PC1 Hosting (cloud, public), PC2 joining - running for approx 4.5 hrs until it lost connection.
Test 2 - PC1 Hosting (cloud, public), noone joining - running 9 minutes until it lost connection (10-25 min seems the norm)