advice from DCS support was:
"for logging to Module Manager should be opened an access to http://www.digitalcombatsimulator.com with port 443
for logging to Multiplayer game (master.digitalcombatsimulator.com) should be opened port 5222
both connection uses SSL/TLS."
I assigned those ports to my router (games and applications) and I used port forward network utility to check ports. All but 443 on TCP protocol are open. (Have in mind, I tried all this also with firewall completely off.) It's odd that 443 is not opened, but it's for Modul Manager which I think don't need since I use steam version, not standalone. From what I understand, 5222 is for server which I can't access.
Pingtest - turned out ok. No losses.
Ipconfig /flushdns - tried, but no change.
Switched internal IP to static, to remove that from the list of possible problems. No change.
Now, WinMTR shows some interesting results but I'm not sure I understand them correctly. (attached screenshot). Line 17 - says 63% loss, and it starts recording losses only when I try to connect to multiplayer. So that could be a problem.
I guess ISP is doing some blocking? :huh:
Attempts to connect to multiplayer almost instantaneously shoot to "can't connect to master"
Log:
00021.136 ERROR NET: Can't connect to Master: SSL connect error
00021.136 ERROR NET: Master login : 503
00023.782 INFO NET: client has stopped
I guess I'll have to call ISP, but in meantime I will see what DCS support has to say.
Also, I'll reinstall the game completely over night and see if that helps. Since I did integrity test, that would be the dumbest solution for the problem, but I'm willing to try it.