I just went through this upgrading my fairly simple blueprint function library from 5.4 to 5.5, even having to go so far as to just create a new C++ project and then cut and paste code…
The error messages are often completely useless, but I switched to the output view after the build fails and follow it up until I found where the actual errors are. Things changed from 5.4 to 5.5 and I had to figure out a new way to accomplish what one of my functions did. After fixing that, it finally compiled.
I got the code 6, code 8, and the bug where the unrealbuildtool mysteriously just vanishes and needs to be rebuilt still is not fixed, so I actually have a shortcut to that folder to rebuild it when needed.
Long story short - I don’t know if this is your issue or not, but there were errors in my compile that got lost in the sea of over 50 things being rebuilt. Once I found and fixed them, it worked OK - but it was after creating a new, empty plugin with the latest version and then cutting and pasting my old code.
I don’t think I’ve ever been able to simply recompile when upgrading, even when my specific code dependencies hadn’t changed.