I think I know what your problem is. I had the same problem. A3 support were useless and I figured it out myself. (I got an email saying change your password and check...and then nothing!)
Anyway, if this is what is happening, then you have the same problem as me..
While questing, the game froze and had to be quit (ALT+CTRL+DEL) When I tried logging back in, it would say 'invalid login/password) I checked logging in on the forums and that worked perfectly. I was also able to check my profile and account usage. So, my account had not been hacked. Once the above is the same for you, then try out this simple solution...
1. Uninstall the game.
2. Delete (shift+delete) the folder where the game had been installed.
3. reinstall the game in a new folder.
You should now be able to login without issues.
I have found that apparently the game also creates some rubbish temp files which at times causes it to CRASH! or lag. Again, the only solution to when this happens is to uninstall, delete the folder and reinstall.
painful but it works for me...and saves my -2% from lag as also the problem of my stuff dropping in maps (hat eit, happened twice due to lag!)
Hope this helps...
cheers!