I have been converting projects all day from 5.5.1 the best thing to do is to reset the project file and turn on all the plugins and reset the engine back to default as much as possible.
So edit the project file the icon which contains all the plugins on and off is a just text file. You should back up this file and be careful while editing you can make it so it won’t launch the project if you get a bracket or comma off.
Then go into the project turn on all the plugins you need and leave everything on for now.
See what errors you are getting after this process. Things you have on and or off or 3rd party plugins might need to be compiled for 5.5.1. Since it was all running fine on 5.4 it shouldn’t be too bad. One thing can cause all your issues.
Once you get it up and running you can start to go through each option and start manually removing each thing you think you don’t need until you get it back in shape. I noticed a lot of the things I always had off for years now are required by FAB or Bridge which is annoying.