Plan on addressing errors in a timely fashion and issuing new Beta versions for continued testing for weeks or months. The more serious it is, the faster it should be addressed. If it's a real showstopper and you don't get it fixed quickly, you may lose your beta testers, who are also future customers. You probably want to fix those and get an update out while you work or wait for some clarification on others. If it was 2 days ago, even if you have fixed a dozen more minor issues, you may want to wait a little longer.
Beta testers can get pretty frustrated it there's a new version every couple days.
Don't wait too long, though, that frustrates them, too. No more than once a week and no less than once a month are good general guidelines. Which platform and operating system the problem appeared in. These need to be addressed as well. These are errors or bug reports that simply cannot be reproduced.