AECOsim SS6 Custom Linestyle Prevents AutoCAD Files Opening

Still trying to get some time to work through this one, but I have a couple of custom linestyle libraries that I used for detailing rebar that, for some reason, prevent me from opening AC files. No warning, nothing. Take the files out of the build and all works fine. 

Before I pull them to pieces, has anyone come across this sort of thing before?