Site running fine, then sql access denied with correct credentials

hello drupalers,

I have a site that's been running fine for a while. It's a bit of a sandbox site so I'm not sure exactly when things went wrong, (around the last drupal update?) but it suddenly started giving me:

The mysqli error was: Access denied for user 'mysqluser'@'example.com' (using password: YES).

even though the credentials are correct. How do I know they are correct? b/c I looked them over carefully, my ISP was kind enough to check over it with me, I tried them from the command line, and I'm even able to set

$update_free_access = TRUE;

and run updates (this doesn't work when I change the credentials), but I still get access denied. I have tried obvious things like starting with a fresh settings file and so on, so I don't think it's that.

I'm not sure what else to ask. Is there somewhere else this could be failing or getting mucked up? something I can fix? It is a sandbox site so I could drop it, but that would be a fair bit of work. If finding the issue here is also going to be a fair bit of work, I guess I'll drop it, but, WTF?