I've been having the same problem as the OP for two weeks or so.
My log:
http://jdownloader.org/pastebin/3326
I have a Hotfile Premium account valid until 19/06/2010.
When this error occurs, the premium account settings in JD show that my account is invalid. Refreshing the account makes it valid again, and then I have to enable premium for this hoster again.
I have the same error when logging in from the PC soon after the error occurs (Too many login attempts...).
Edit: I have rolled back to Java RE 6v17 (before the weekend) and the problem still occurs. I'm guessing it's not related to Java.
Edit:
Max Connections is set to 1;
Let Reconnects interrupt resumeable downloads is ticked;
Do not start new links if reconnect requested is ticked.
I've kept logging enabled for when it happens again so that I can post more info if needed.
Hope this helps (and thanks!).
Cheers,
Andrew