"Save for Web" - not working (Photoshop CC 21.0)

"The operation couldnot be completed" it says I get this strange error message, stating: "The operation could not be completed. An unknown operating system errror has occurred" as can be seen at the attached file.

Is it the Mac machine or is it the software?
(according to me it's a photoshop issue - can't be "än operating system error", but hey... im here to ask - not give opinios)