This seems to be a recurring problem with Yahoo Mail Beta. I too have experienced this several times.

Sometimes, I can clear all browser cache and Yahoo cookies files and the problem goes away. Other times that does not work.

I also fixed the problem before by upgrading my Java version.

Unfortunately there is not an easy, cut and dried solution to the problem. Most everyone here will tell you to simply clear browser cache and history files and reboot. Honestly, that does work sometimes, but not every time.

You might also check the help files for freezing browser windows and other error messages here…

http://help.yahoo.com/l/us/yahoo/mail/yahoomail/technical/index.html

I routinely switch back and forth between regular Yahoo Mail and Yahoo Mail Beta. Everything you need to know about switching back and forth (like I do frequently) is here on the bottom of this Yahoo Mail help page…

http://help.yahoo.com/help/us/tutorials/cg/mail/cg_migrate1.html

Also, Yahoo has reported that the Beta period is over for Yahoo Mail. They are rolling out the final released version over the next few weeks. See the Yahoo blog for more info…

http://ymailupdates.com/blog/

i get that too theres an x in the right corner of the error window just close it and you will see your e-mail