This may sound like a silly question but are you attempting to sign on multiple devices at the same time?  Is there a limit as to how many devices you can be signed on to a site at the same time? 

I would try it but I only have a desktop PC so I'm of no real help, sorry.

No, mate, I wasn't signed in on another device at the time.  When I was trying to login via the media player browser, Firefox on my PC was closed. Same when trying to login on my tablet and 2nd PC, several attempts were made but all were unsuccessful.

I should probably have mentioned something about 3 weeks ago when I had issues logging in.  I tried several times to log in from the home page without success, but when I went to the forums to see if here were any new posts/replies I noticed that I was in fact logged in.  Now if I pressed the back button I was logged out again, meaning I had to press the home page link to remain logged in, then navigate to other pages from there.

No worries, mate, I'm sure somebody will come along with a solution/answer.

There's no issue with being/attempting login on more than one browser or more than one machine.

Tracking cookies need to be made but if there's some blocking privacy proggy running it will prevent it.  forums.wincustomize.com and wincustomize.com both need to be 'trusted' re cookie creation...

There are no privacy apps running on the tablet or my 2nd rig... none when using the media player browser, either.

I'm not too sure about cookie creation on the media player browser, though.  I would have thought it was done automatically, but I will have to check if there is even a cookie cache in which to save them... though it wouldn't make a lot of sense not to have, or not to be able to log into frequented sites. 

Thing is, I had issues not being able to log in with my tablet and 2nd rig... and about 3 weeks ago on my main rig before retaining the cookies and bookmarking with myself logged in.  Now that instance was peculiar.  I had discarded all cookies and bookmarks to freshen up Firefox [no privacy apps were running] and 4 to 5 attempts to log in appeared to fail. However, when I went to check something in the forums I was in fact logged in.  Shaunna had similar issues logging in about the same time. 

They say stranger things happen at sea... but I'm not so sure.

Over the last few days I have been repeatedly logged off mid session and unable to log on again for quite some time....

happened in last hour ....took about 5 log on attempts to get back...

I essentially never log off....[almost like I don't have the option... ] ... but if I find myself logged out at any time it is always due to site/forum server resets...or forum updates.

Over the past few weeks T-Man has been 'twiddling' or 'tinkering' ...so there's been a bit of that...

Perhaps that's the cause/reason... and hopefully so.  Means it's not on my end.