fix: mismatch between extentref entry reference count
Is this a serious issue? Maybe...

It is an issue with your disk that needs to be run in an environment where the normal boot volume is not the active startup volume. Fortunately that is built into all recent Macs.

FIRST, you have a recent backup, right? If not and you can make a backup, do so now.

Now boot into recovery mode by holding the ⌘ Command R keys during a reboot. Eventually you will get to a screen (if memory serves...) that asks you to pick a language and then you get the recovery environment.

Go to the (I believe) Tools menu and select Disk Utility. Now select the HD that you boot from and click on repair.

Depending on what is wrong and how fixable it is this might take a few minutes. It also might take an hour or more. Let it do its thing. Hopefully it can repair the error.

Note: as the OP mentioned sometimes running Disk Utility a few times will fix it on subsequent attempts. So try it again, and maybe again and again.

If it can't repair the error... Well normally I would advise using DiskWarrior (or other disk repair utilities) but I am not aware of any 3rd party disk repair utilities that work on APFS. Perhaps someone else in the AskDifferent community knows of one...

If there isn't one you basically should check your backup (make sure you have everything) and then reformat the disk and reinstall from scratch and then restore your backup. Why? Well if you have a disk error that Disk Utility can't fix and there are no 3rd party utilities then all you can do is reformat, reinstall and restore.