Glad to have found this topic this morning, as EVERYTHING!!! went nuts last night and I received many angry emails from clients. Basically, any website where a phpBB forum is part of the environment, the issue mentioned on this topic is clearly happening, where you log out and it randomly logs you in as another account. What is disturbing, is checking client forums (other than our own) and I appear to be logged in as one of their members (until hitting index.php).
What is really strange, is that header graphics and website backdrops are affected as well, again, only on websites where phpBB forums are present. To make things even more confusing, "some" issues are resolved by using Chrome or Firefox, against IE 11 or Edge.
Not a good night for me last night and still no fix after talking with my host company, who of course, focus on a local issue (clear your cache bla bla bla).
All the best,