I had this bug creep in as I migrated from 4.6.1 to 4.7.2 overnight … It compiles all blueprints and runs fine, but wont save a certain function library. For me root of issue was inside same blueprint function library that wouldn’t save. I tried stuff mentioned above such as copy paste to a text file and looking for “transient” etc. no luck. there was no faulty reference to be found.
What worked for me was to methodically delete groups of nodes in print functions and save until I identified which node(s) were making system crap out specifically.
In my particular case, one function was calling into another within blueprint function library so I deleted 2 nodes in question and added them back in and rewired them and then it saved properly. It seems to me like some sort of reference error when project migrated versions. perhaps there is an edge case with checking references that is not being handled by blueprint compile.
I was hoping to undo and copy paste faulty nodes and working nodes to notepad and compare but I’ve already saved over it and it cleared undo history. I made extensive changes so I cant easily revert right now. Perhaps someone else reading this is in a similar situation and can catch difference so epic can fix it or hopefully my description reveals something.