You may wish to check a setting on your old PC and compare it to the new PC. It is experimental, and I don't know how well it works, or its development status, but there is a setting for long file names in MC.
"Options > General > Advanced > Support extra long file names (experimental)"
If that was turned on in your old PC, and is off in your new PC, that would explain the failure in the new PC.
Perhaps try turning it on if you haven't fixed all the issues, and see if that setting does it.
PS: I agree your music shouldn't be under your User Profile directory, or on your C: drive at all. It would have been best if you new PC had a second hard drive in it, and the music was put on that, or if that was a problem, the music went onto a fast external USB drive. But as you have put it onto your C: drive, keep an eye on free space there. You should never let it get below 10%, and preferably 30% free space.