Okay so I have this problem for 3 days now. But I tested with my partner and they can log in on my account no problems. I can log on an Alt no problems and their account with no problems. But only specifically when I log on this main account I get a message saying I am already logged in. And when I have my partner logged on that same account and then I try to log in I correctly get a message saying I am already logged in and it is logging me out. It also logs my partner out when I do that. I have done the whole restart router, restart pc, reinstalled firestorm, tested slviewer. Still same account I cannot log in on. Same error always. This is getting annoying. I messaged Linden and they said it might be internet in my area and that this is a rare error that happens blah blah. I left it a day then messaged them all this stuff I have tried. They haven't bothered to get back to me...
P.S. I did a DNSflush too...so really i tried everything. It's really rubbish