I'm using a PC running IE8.
I try to login and I get this message:
Please log in or register to view or modify your profile.
I look at the site backend and I see the user is actually signed in.

I try to use IE8 on my MAC (virtual) and it works fine.

All other browsers work fine as well and login as they should.

I tried to go to Safety and delete all cache and data from the PC IE8. It doesn't help. I tried to disable SEF completely and still nothing.

You are probably thinking "Well, stop using IE" or "Well, if it works on another machine running IE8 then it's probably your computer's fault". I would agree with both statements, but it doesn't make this problem less of a headache and less real. Users facing this issue will not spend time trying to figure out what is wrong or what to do.
What can I do about it?

I want to add somthing to my post.
I performed a system cleanup of temporary internet files by going to Start - Accessories - System Tools - Disk Cleanup and then erased the temp internet files.
That fixed the problem.

I seriously hate Internet Explorer... so much.

For now yes. I just fear tat this problem my occur again, or happen to members who are not going to bother erasing temp files just to be abel to access the site.