Initialization–Database couldn’t be created!

Which message reappeared - the in use or the initialization failed message?

Both, in that order

Where are the databases located - the file path on the machine?

/Users/my_username/Databases/

Removing the DEVONthink.lock file should resolve the initialization warning.
I PM’d you.

Na, that was the whole problem. I had deleted the lock file from each database; each database opened without error so long as I only opened one database. As soon as I attempted to open a second one, I encountered both error messages (in use and failed to initialize) for that second database. The only way to successfully open more than one database was the convoluted method I described above (and I’ll get back to you re the PM)

There is no file DEVONthink.lock. I cannot open my database

Are you running DEVONthink 3.5.2?

yes, that is right

Close this and change the extension back to .dtSparse.
What happens when you double-click the dtSparse file?

Screenshot 2020-08-20 at 17.36.23

and it becomes .sparseimage

Did you rename the database or the disk image? You should have renamed the disk image.

yes from . sparseimage to .dtSparse

Screenshot 2020-08-20 at 17.40.55 Screenshot 2020-08-20 at 17.41.30

when I open .dtSparse, it changes to .sparseimage

The database inside the disk image should be dtBase2.
The disk image should be dtSparse.

Screenshot 2020-08-20 at 17.42.59 Screenshot 2020-08-20 at 17.43.11

Screenshot 2020-08-20 at 17.44.05

Press Continue.
Choose File > New Window, if needed.
Do a File > Verify & Repair on the database to ensure it’s intact.