
Either way, if any one of you gets the chance to, please try the default SMF theme if it has similar login issues.
Because I'm fairly certain Goober removed the session modifications associated to the old mainpage we haven't used for ages and double checked it himself. Then there are no such bugs reported anywhere on the official SMF forums or bug tracker. So that leaves the Sandwich-cooked HLP theme, which I really don't think should cause such issues, but you never know.
But most of all, it's really weird that these issues seems to be limited to a minority. I had them when the session modifications were still in place, but since then no issues. It's beyond me why some people still continue having the very same problem despite the session modifications have been long removed.
At this rate I don't see any other choice put to remove all modifications, including HLP theme and see if problems still persist.