[PAYH] Plesk login and Firefox

[#1]

I've been going batty trying to login to Plesk - I know I've been entering the correct username and password (I've copy pasted them). Each time it returns to the login page - but there's no error message to say the login failed.
Out of desperation I tried IE instead of Firefox - got in first time. But at the top of the screen it says:
Warning: Another user with the same login (elvin) is already logged on to the control panel. See the list of currently logged on users.
When I click the link I get the attached screen - i.e. the Firefox logins had partly worked but just not taken me to the correct screen.
When I tried Googling this problem I quickly found two or three references to this identical problem with a solution of turning register_globals on. This doesn't seem a particularly good solution to me.
Anyone any ideas?
Edit: Safari also got me in first time.

[#2]

If you have a plus.net PHPSESSID cookie, try removing it and logging in in Firefox.
I had problems with the usergroup.plus.net cookie domain conflicting.
B.

[#3]

That's why I like these forums!
Thanks Barry.
Do either PAYH or usergroup.plus.net need the domain of the cookies changing?

[#4]

I already raised it on the PUG forums, but don't recall what the outcome was supposed to be.  Let me chase it
B.

[#5]

I've done some more testing and confirmed absolutely that it is the PUG forums that cause the problem.
I've raised a ticket as I don't think this is an acceptable situation!

[#6]

Until this is resolved I wonder if something about it should be added to the FAQ?

[#7]

You could use this as an interim measure?

[#8]

Now I know what the issue is it isn't such a problem for me - just delete the cookie and all is well.
I was just concerned that nobody else wastes two hours figuring out what the hell is going wrong like I did.

[#9]

I find that if I click the 'Manage Mail' icon in Firefox then expand asks me to login again (and refuses to accept my details). This doesn't happen when using IE. Perhaps related to the same thing I wonder?