That error message is just saying you weren’t able to connect with session, probably as a result of a mismatch as described above. Judging by log, I’d say you’re still only attempting to join one session for some reason. It might be worth exploring a more thorough approach if you keep running into this problem.
I’d take a look at Multiplayer Shootout example. There are some issues with UMG setup that makes it not work all time (due to a change in how widgets are handled between levels between 4.7 and 4.8), but if you open up logic used in widgets you’ll see how it works. Basically, it creates list of each session found and allows user to select which one they want to join.
Alternatively, you might try creating an array of found sessions, then call a custom event. In custom event, attempt to join first session in array, and if join fails, call event again. Throw a counter int variable in there to track where you are.
Again, since this seems to relate to a problem with dev id being used across multiple users, this probably won’t be fixed anytime soon if at all since having your own custom dev id will solve that in actual game. For testing purposes, you may just need to work around it. However, if you’d like me to test project out on my end to confirm that it’s same problem (I don’t run into problem here), I’m happy to do so.