Can't connect Mac-to-Mac with Catalina update

I have an iMac and a MacBook and have never had problems with connecting one to the other over wifi. Suddenly, I cannot connect them.

• I have checked to make sure both are listed in my router's software as being on the network (the iMac is wired to the router, the MacBook is wireless, but both are showing up on the list of devices connected to the network).
• I have the right boxes checked in the Finder preferences.
• On each Mac, I can see that the other computer is listed under Finder> Network - so the computers each "know" the other is there. But when I click on Connect As (registered user) and enter the other computer's admin password, I get Connection Failed.
• This happens in both directions (either computer to the other).

Any ideas? Grateful for any help. I use the iMac as a Time Machine backup for the MacBook, but that isn't working now since I can't connect them anymore.