Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
If you like to introduce yourself, to the community, go here:
http://lmms.io/forum/viewtopic.php?f=4&t=4480
All programs with complex file/ folder structure has issues with migration from one pc to an other. That is typical a big thing. In LMMS migrations is next to frictionless if ....IF you have used default installations on both precious system and the new one, and only then!
If you have installed differently, then you have issues!
The way ahead is to install default on your new system, and then manually move all old resources from your previous pc to the corresponding new folders
But you will still need to manually edit every Sound-font path in the project-files.
My approach has been to only edit a file if i wanted to edit the file, if not i have just left it alone.
Bottom-line: Always install default !