No Photo Stream in 1 of 2 iMacs?

1 of my 2 iMacs is getting My Photo Stream (from an iphone and an ipad) just fine, the other isn't, no matter what I try. Same AppleID, same everything.

This problem started exactly after the Photos app replaced iPhoto in the MacOS, or after an OS update, around April 23-24.

THe iCloud shared albums I can see from my iphone, ipad and one of the iMacs, not in this one...

Has anybody had the same problem?