Thanks Dave,
ok, what you can do is troubleshoot the Control Center while you try to save again the ldap source after changing the password:
1) enable DEBUG logging level on control center via the following CLI command:
$ cc-config --log controlcenter --level DEBUG
2) reproduce the problem (click save on ldap source page after testing query)
3) have a look at the BirghtmailLog.log (diagnostics command can package it for you) at the same time and look for any error you can spot there.
4) Hopefully you may search our public KB for that error code/message and find possible next steps for your troubleshooting, otherwise please post here the cc-log snippet so I can have a look.
Regards