This is what happens when you have multiple versions of the same program installed, and we don't even know the type (Installer, Store, Portable). For example, the Installer versions will use the same folders.
Though you say you ran an uninstall for 3.4, there are probably pieces of that and 3.5 scattered all over your PC and registry. At this point forget about the "settings" file, and use your mistake as an opportunity to do a clean install of 3.6. The key point being, you HAVE the music files, thankfully. Be sure that they are backed up in a separate location that has nothing to do with MusicBee, and do not use that word in the folder name.
First check the Add/Remove Programs in Windows and run the uninstall for any "MusicBee" entries still there. Then manually do a search for any file or folder with "MusicBee" anywhere in the name, preferably using a direct file table searcher like
SwiftSearch64 , and delete what you find. Finally, restart your PC.
There probably will also be registry entries left over but rather than wade into that we'll avoid it altogether by installing the PORTABLE version of MusicBee 3.6. Install directions for the Portable version are here:
https://getmusicbee.com/forum/index.php?topic=40059.0 . Remember to finish up with Step #7 to get the latest update.
Now you can start over clean, creating a new library from your music files. Doing the settings over will be a good learning experience to find all the new options you may have missed before. And because now all your MusicBee settings and files will be in a single folder, backing it up will be as simple as a 2-step folder copy operation. Take advantage of that to try new layouts, knowing that if you end up down some rabbit hole returning to the last good copy is as easy as copying back your Portable's backup folder to give things another shot.
Note: please resist the temptation to let MusicBee "auto-organize" your music files at the new install. This can be quicksand unless you know exactly what you are doing. This operation can be done afterwards at any time, so if later you are still interested take the time to read the multitude of forum posts concerning it first. This custom forum searcher by hiccup is your friend:
https://cse.google.com/cse?cx=426af386625354430#gsc.tab=0