I tried to merge this into a new thread, and I'm not sure if I was successful. Anyway, JoshTheBlack
found out a fix that is working for a lot of people's forum issues of late, since most of them seem to be affected by google chrome. It seems recent updates to chrome's default security policy have caused this issue. See the next comment for details, but the workaround/fix is this:
Type "chrome://flags/#schemeful-same-site" in to the address bar.
Set the "Schemeful Same-Site" flag to disabled.
It will tell you to reload, so press the reload button.
Try to log in.
Jim and I are using this information to see if we can make a simple tweak to our SSL cert on the forum server to make it so you don't have to do this workaround, now that we know what the root cause is. Thanks to all that helped us track this down over the course of the year it has been happening.