What version of MB are you using? (Help > About)
How was MB installed? (Installer, Portable, Store)
If you're not using 3.4.x, then I suggest updating to it first before trying anything else. See my signature.
If updating doesn't change anything, then using a file explorer find the playlist in question. Then open it with a text editor and see if if looks correct. If it does, then tray another playlist format that Rocket Player can accept.