OK - I thought I had the answer but the problem of being directed to my website front end instead of the SPPB edit screeen. But it's happening again.
I created a new module, selected for SPPB, chose a position and clicked on edit in SPPB. Now I see the website home page and a message at the bottom right corner of the screen: "You don't have permission to access this. Please contact a website administrator if this is incorrect."
This time Global-checkin does not report any items needing to be checked in.
I've also enabled Joomla error reporting but no errors have appeared.
However, I can open SPPB pages I created yesterday. Just not the one I'm trying to create today.
I'm logged on to the back end as superuser.
Waiting here patiently (probably on the other side of the planet) wondering what the Joomshaper support people have to say.