I can't see special problems with troubleshooting guide.
We also rebooted all the 3CX server completely twice.
I can confirm it's a user profile related problem: only some users are affected.
From the same PC, I open chrome, logon as affected user, and the problem appears. I logoff from webapp and re-logon with another user (without closing Chrome browser), and the second user can access webmeeting with no problem.