Because it was claiming an IllegalStateException, and it was pointing to the sendRedirect() statement, I assumed the sendRedirect() was the CAUSE. So--
I started probing the code by inserting
response.sendRedirect(<page_name>); return;
By putting this pair of statements in various places, I was able to deduce that the problem was occurring in a subroutine that accessed the MySQL database. Furthermore, if only the first part of the subroutine was executed (as it would be with an invalid login name), it worked, but if the full subroutine was executed (valid login name), it failed.
At that point, I noticed that if I clicked on the brower's Back Button, the sendRedirect() that I had suspected of being the culprit had actually worked. Hmmmm. So, I tried simply inserting extra return statements in the code rather than the sendRedirect()/return shown above. Lo and behold, I got a SQLException from the suspect subroutine I had been probing.
Shortly before the problem started occurring (remember the "suddenly" part of my message subject?), I had removed a certain field from the database table, but I was still asking JDBC to get() that field value. That, of course, was an error (MY error) and fully worthy of an exception, but the question was why did it cause the error it did.
I apears my error handling needs some work. I'm going to post a separate question about catch{} to try to find out what I'm doing wrong.
In the meantime, thanks to Howard and Ben for their offers of help.
Merrill
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