Mail keeps rejecting my .mac password and won't retrieve my messages - why?

Mail has worked flawlessly for me for several years. But this week, it stopped loading (retrieving) my messages from my .mac/me/iCloud service.

For some reason, my .mac account in Mail is offline. When I try to take it back online, mail tells me that my password isn't being accepted. (But it works just fine if I go to my iCloud account and use it to log on and check my mail there. And it's no prob retrieving my mail on my iPhone, either.)

Can someone point me in the right direction for fixing this prob? (BTW: I have several other e-mail accounts set up in Mail, and they work just fine. Only my .mac/me/iCloud account stopped working.)

I've tried to change various settings in the preferences for the account, but it's been to no avail.

Some of the various error messages I've recieved after trying to fix the prob:

And here are info panels I see when I check the account in Mail's preferences:

Note how port 993 is greyed out here; it's not greyed out for my other email accounts (and it's the same port w/ those, too):

Any feedback would be greatly appreciated. Thanks in advance for the help.