I've found a little bug... However it may be an issue with signing in using a Google account rather than an actual forum issue.
If I'm logged in on the forum using Chrome on a Windows 7 pc, then log in using chrome on my phone (Pixel - V 7.1 ) if I try and navigate around the forum on the pc I then get this error -
I can clear the error if I then log out of the forum on both android and pc, then reenter the forum fine.
Granted there is no real reason I need to navigate the forum on both devices at the same time. But it probably shouldn't be doing it, other forums don't.
I've had the 500 error a couple of times too, but generally reloading it clears it for me. Figured the server was simply busy as there were multiple new posts when I refreshed the page.