I got the same problem, I locked myself out of my backlight 3 backend.
In my recent log-file, I cannot find any string like the one above.
The most recent line starts like this:
[07-May-2020 15:58:20] ERROR: join(): Passing glue string after array is deprecated. Swap the parameters
I remember initially having duplicated an existing backlight-Project to not being forced to start from the very beginning… could this be a problem? Any way I was still logged in and working on the new site last year. Unfortunatly all password combinations failed… althought I had them written down.