Take a look at ParamStr(0) or Application.Location
How can I check them?
Place a TLabel on your Form, and assign ParamStr(0) or Application.Location in Form.OnShow.
Another Option might be to Write to a txt-File inside the lpr-file before the Form loads
It's "old school"-Debugging.
Just thought of something else, since you mention OneDrive:
I remember falling afoul of a OneDrive-location, since OneDrive actually uses Forward-Slashes as DirectorySeparator if you don't use absolute local path.
That said: Try
./DataBase/DB.FDB
as relative path
On a sidenote: OneDrive has given me blinding headaches in the past.
In a nutshell: During development i read the Registry under
Computer\HKEY_CURRENT_USER\SOFTWARE\Microsoft\OneDrive\Accounts\{Profile}
with "{Profile}" being whatever Account you use (Business, Personal, whatever), and INSIDE there it's the Key "UserFolder" which gives you the local path.
Construct full LOCAL path from there (With Backslash as Separator)