Same issue. MacBook Air M1 2020, macOS Big Sur 11.2.3--I was able to make a back up literally yesterday and I haven't changed anything about any of my software.

I tried verifying and rebuilding the data. I tried rebooting my computer. I tried unmounting and remounting the external hard drive that I back up on.

Even when I go to https://help.quickbooks.intuit.com/en_US/contact, and even though I am still within the time frame of 6am-6pm PT, it says Open Now for Message Us but when I try to Start a Message it says "Our experts are currently offline. Please try again later."