Thanks everyone for your replies. I'll try some of the things suggested here, including reinstalling the browser, although everything else seems to be working perfectly fine.
I'm using a laptop at a single location. I'm not moving around or using a phone to connect. Just here in my home the same way I've always logged into TERB and the internet connection is working perfectly well for all other websites. No protective or battery saving apps, and no ad-blocks. I have always run an anti-malware app and everything seems to be fine there. Nothing has changed at all about how I run my computer, and again every other site I log into works perfectly fine... TERB is the only website that won't keep me logged in.
The problem manifests itself in various ways. Sometimes when I'm first trying to log in I get the notice saying "thank you for logging in" but then instead of taking me to the forum it just takes me back to the main page again as if I never logged in in the first place! Then I try to log in again and the same thing happens multiple times in a continual loop. Eventually finally I get in... sometimes I get in just by clicking on the TERB logo and presto it takes me to a page where I am already logged in... very strange!
Other times I'll be already logged in and surfing around but when I'm just jumping from thread to thread I keep getting returned to the main page or getting messages saying I can't access something because I'm not logged in, so in those cases I'm somehow getting logged out after I'm already in, whereas in the first case I'm just logging in in over and over trying to get started. Needless to say when you're trying to turn pages or jumping to new links and constantly having to log in again and again it's pretty exasperating!
I'll try clearing the cache / cookies although I do this pretty regularly and that doesn't seem to help. Like some others I never had any of these issues until TERB was closed for a few days just recently, then all sorts of unusual things started happening after that.
When I can log out of TERB normally I get an "all cookies cleared" message regardless... maybe it's just saying that and not actually doing it? In any case I'll try clearing my cache and cookies the next time it happens and see if that helps.
