Viewer failure

I’ve been having login failures off and on over the last week. After I use the Imprudence viewer to log in to InWorlds or OSgrid, I sometimes then get login failures in all the Second Life viewers, for all accounts. The login progress bar starts, and then this error message pops up:

Login failed.
Sorry! We couldn’t log you in.
Please check to make sure you entered the right
*Account name
*Password
Also, please make sure your Caps Lock key is off.

I’ve uninstalled and reinstalled both the Imprudence and Second Life viewers multiple times, being sure to eliminate the logs, the caches, and the “application support” files. (I’m on a Mac.) I have been able to get SL running again by using a backup copy of user settings, but I don’t know what it is I did correctly when I did so, because I’m currently not able to access SL even after redoing the entire reinstall again. I can’t even log in to SL with Imprudence (which I could one of the prior times.) I’m really stumped.