Correct Apple ID & password gives me an error and won't let me run an app I bought off the Mac App Store

Older iMac running OS X El Capitan 10.11.6

I use this old iMac for invoicing software that wont run on the current os. I've been doing this for years. App was purchased off the Apple Store. It's been doing fine until the last few days. No changes to the computer or OS.

I open the app and it asks for me to sign into the App Store. First time it's asked me to do that since purchase. I enter the correct info and get the error "An unexpected error occurred while signing in. The data couldn't be read because it isn't in the correct format."

I tried opening a different app from the app store - same problem.

I also have a virtual machine on my newer iMac running the latest OS - virtual machine is running the same OS and program as the old iMac. Same problem with a different error when trying to sign in: "plist parsing error"

I've notified the app store of the problem - no reply for a few days. I need to get into this program as I can't send out any invoices.

I can sign into the app store on the current OS just fine.

Suggestions would be appreciated!