MacOS Safari iCloud tabs stop working randomly after some time
I managed to find an easy workaround which doesn't require to terminate the user session.


Quit Safari
Open Activity Monitor
Type in the search box "safari" to filter the processes list





Quit and Force Quit (where required) every user process matching the "safari" search
When the Activity Monitor list is completely empty, open Safari and wait some seconds


The iCloud tabs button should appear enabled again.