Big Sur "already open" problem

Every morning I am getting the warning that my principal data base appears already open, when it’s not—or at least I don’t see it.
Here’s my routine.
During the day, the data base window often disappears after i haven’t used it for a while, when I don’t recall having closed it.
I don’t shut down or close the data base all dayall day, but there might be micro current outages.
When I shut down, I always choose to reopen everthing on startup.
DevonThink opens on startup, but my database window doesn’t open.
When I try to open the database, I get the “already open” warning. I open anyway. Log says Data base successfully verified.
Running DT 3.6.1 on iMac 27 standard 2017 Big Sur 11.1
Arc makes a daily backup to Google Drive. Backup log is normal.
I didn’t have this problem before Big Sur.

Where are you storing your DT database files? Do you use sync services, if so which ones?

This to me sounds like something is causing DT to unexpectedly lose access to the database file, I saw something similar when I had accidentally mounted a copy of the DT database from a network drive rather than locally or if the “local” drive is unreliable.

My database files are on my iMac internal hard disc.
My sync Store is on CloudMe. No problems for more than a year.
I had turned sync off for about for about a week and turned it back on today.
I am wondering if there is a problem with my Arc backup. I will see if the database shuts down after the backup tomorrow.

I also have a Time Machine backup of my database on an external hard disc, but there are no copies of the database that could accidentally be opened by DT.

Try quitting DEVONthink before you shut down your machine (which is generally a good idea anyways).

Does it quit cleanly?

Thank you, Bluefrog.

Last night, I quit the computer as usual with the database window open.
This morning, I started up as usual. Devonthink opened with the database—no problem.
Sometimes when four or five apps are competing to open on startup, an app called “Clean my Mac” reports that Devonthink is not responding and offers to force quit, which I decline. This morning, DT was one of the first to open and did not get stuck in the early morning rush.
My regular Arc backup took place at noon without problem.
After receiving your suggestion, I quit Devonthink, apparently cleanly.
After a few minutes, I reopened DT with the Database and again received the already open warning.
I reopened DT with the database, and finally did not receive an “already open” warning.
All this is minor but annoying. I have rarely received the warning before Big Sur and each time I hesitate to click continue for fear that I may break something.
Apparently the solution, is to close the database before shutting down, and to reopen it individually after startup.

Note: We do not advocate opening an application like DEVONthink on system launch. The fewer items that open on login, the better.

When the database is open, run a File > Verify & Repair on it. What is reported in Window > Log?