Sparse disk bundle weirdness in Big Sur

Another update:

I've had some succes with an encrypted sparse disk bundle created on macOS Mojave (as opposed to Big Sur). At first I thought there were less "file can not be opened"-errors because the image was new but even after several days, there were less problems then with the sparse bundel created on Big Sur.

Right now I haven't had issues for at least 48 hours.

There are, however, two more variables that might be at play here. First: the volume of my present image doesn't have a custom Finder icon when mounted (whereas all previous volumes did), and second: I've installed the Big Sur 11.4 update.

Good to hear your issue is solved. I still have issues with files sometimes not opening (or not able to be saved) but nowhere nears as often as before.

AFAIK the situation on my MacBook Pro got a bit better after installing the Big Sur 11.4 update, but what really helped was quitting the apps that opened the files in question, right after use. Or at least before system sleep (I have no definitive proof that sleep cycles are a factor, but I had no problem opening from and saving files to the encrypted disk image, directly after mounting. Only after several hours, issues would occur, which most times included at least one instance of system sleep).

A small update:

I have been able to confirm that the sleep cycle is (most probably) not a factor. I've mounted my sparse disk bundle and opened a Numbers file from it, kept it open while working in other apps and when, after about an hour, I tried to save changes to said Numbers-file, I got the 'Unable to save" error message. My Mac didn't sleep nor had it's display shut off during this time.

I had to save a duplicate of the Numbers file, quit Numbers, Force eject the sparse disk bundle image, remount it, open the file I was working on, and copy-paste the changes from the duplicate.