can’t empty inbox

This is a new problem in past week.

Before now I could empty my inbox swiftly and easily w/o having to click on each message individually (pain!).

I just selected the top message, hit "control A" (for All, I guess), and voila! - an empty inbox.

Now, that maneuver no longer works and I get a jarring sound and the message "An ukn error has occured."  Say what????

Does anyone know why it's doing this now, after working so perfectly?  I get a ton of mail and I can't click on each messageto delete it -  takes too long.

Thank you.