Always use DNS names in a file reference. It is when you mix DNS names and IP values in the file references that versions before (I think, but could be wrong as to the exact version) FMP12v3 could get confused as to who is opening the file.
What ch0c0halic said, but also, do not duplicate External Data References (EDR) from one file to another. You can get this kind of conflict if you already have a file with an open EDR connection to a file and then the same file funs a script or calls a TO which relies on a different EDR in that same file to the same external file. When one copy of Pro encounters two open EDRs between the same two files, it throws a license conflict.
This started to be a big problem in FMP11, and has continued since then. FM Inc seems to have tightened up their networking protocols to catch more potential licence conflicts, and I had to clean up a bunch of redundant EDRs in a 60+ legacy file system I had inherited. No one had ever cleaned up the relationship graphs created during conversion for pre-7 versions, and scripts had been added calling other files by adding a new EDR during the writing of each new script!
I was seeing pseudo-licence-conflicts daily when first moving to 11 until I figured out that those legacy External Data References were culprits.