Hi there,
very simple. On a fairly clean and up-to-date Win10 x64 device, .NET 5 is not installed but has long been replaced with 6(.0.6 - we're talking Desktop Runtime here, of course) by Windows Update. But without .NET 5 additionally installed, on first or any startup, BookLibConnect.exe quits on my machine before even starting, with error message, "To run this application, you must install .NET".
No log file seems to have been written in AppData\Local\audiamus\BookLibConnect\log or anywhere else I looked before 1st successful start, so I'm sorry I can't provide much more initial insight.
Starts up normally after installing .NET 5.0.17.
Thank you for this aweseome piece of software!