Here's what I'd test (maybe you already have):
Are others having this issue or is it just you?
Can you open another database on the server that you should have access to (maybe the nab). Go directly to that url ie: http://yourserver.com/names.nsf
- if you can access, then you know your password is good and your rights to the server are good - then you can focus on your mail file. Maybe the ACL is corrupt on your mail file.
If no go getting to another database:
You're http password was reset or messed up, reset it on your person document.
if you use groups for ACL purposes, the group could be corrupt in your NAB (you won't be able to tell simply by looking at it). Recreate it. Delete the old one, don't try to edit the existing document.
You could be in the deny access group.
You're certificate could have expired on your Notes ID.
You're person doc could have been set to "lockout id"