So there was 2 cases of this on my server and it was really strange so I'm trying to get help on the subject.
Player A and B cant hit each other, but can hit and get hit by player C and D. Both on and off claims and on different worlds, still the same.
They were from the same clan, so I tried telling one of them to leave and even restarting with simpleclans disabled, no effect, they still couldnt pvp.
Yes, my pvp settings are enabled on essentials, worldguard and multiverse-core, difficulty is hard and protect fresh spawns from grief prevention is off (doesnt matter because on nether and end they cant pvp and griefprevention has no protection on there overall). All players can pvp normally.
The hardest thing is that there is no message for them and the console shows nothing top. The problem is I cant replicate it and the two players are busy to test it plugin by plugin (when the bug first happened was months ago and the players who first reported it already stopped playing).