Starting with R41.9 Quicken has a nice new feature to rearrange the accounts in the account bar. You right click on the account bar and select
Rearrange accounts.
When I tried this, I got this error message after I hit the Save button.

It seems that for some reason Quicken is closing the data file and didn't get the lock releasing correct because when it tries to reopen the data file it sees it is still locked and puts up this error message.
Note my data file is on a totally local drive C:\Quicken, with no cloud storage or backup point to the data file.
It is sort questionable that the data file has to be closed just to rearrange the account names, but I will chock that up to the convoluted inner guts of the Quicken code, but it should be able to do this without errors.
After I select OK for the error dialog, I get this.

I can close the "Rearrange Account" bar, but I can't close Quicken (it beeps like there is some kind of dialog blocking getting to the menus, but no dialogs are visible). Right clicking on the Quicken icon in the Taskbar and selecting close all windows doesn't close Quicken. I had to kill Quicken from the task manager.
Well, I just retried it and I can't reproduce the error, and it doesn't seem to closed/reopen the data file. Time will tell if this is a general problem or just a onetime quirk.
BTW it is a bit strange if you want to move an account to end of one of the sections.
You can't move it to the end, you have to move it to the next to last, and then move the last one above it.