I just noticed what seems to be a bug in Maya 2017 & 2017 and took me ages to realize what was causing it.
The annoying issue was basically that I created an nParticle system being advected by a fluid, caching out the sim worked fine but on deleting (Keep files) the cache the issue was with trying to attach an existing cache where I kept getting the error:
"// Error: file: C:/Program Files/Autodesk/Maya2016/scripts/others/doAttachCacheArgList.mel line 65: Must select either nucleus related geometry or fluid objects that the cache will drive // "
I could create a new cache for it but just can not attach one. If I created a fresh new nParticle I had no problem attaching a cache sequence.
So the difference between the two had to be found and the reason was not obvious but the Fluid being attached as a 'field' or not made the difference. On disconnection of the fluid as a 'field' in the 'Dynamic relationship Editor' I can now once again attach an existing cache, but if its still connected I get the error.