So it would be worth testing the local drive versus dropbox just to be sure. Especially since error says it may be "unavailable".

If may be an issue with using a newer version of CADmep than your Navisworks.  Do you have access to Manage 2018 to use with CADmep 2018?  

If could be the the wblock process is not getting the CADmep specific components correctly. It may be stripping this out because it can't read it(?)

Or there is some sort of file corruption going on.

This could also have something to do with your AutoCAD/CADmep coordinates.  In AutoCAD/CADmep, in the World Coordinate System, how far is the model from 0,0,0? Maybe try moving everything much closer to 0,0,0.  Pick a point in the model, such as a corner of the building and put it right at 0,0,0.   Then save and try your Wblock workflow again.

Another thing to check in Navisworks is the Scene Statistics.  Does it indicate that an object enabler is missing?

You may need to install the Autodesk Fabrication CADmep 2017 Object Enabler

However, I'm not sure if that will handle the CADmep 2018 piping. That's why matching release version may be needed.

Autodesk Fabrication CADmep 2018 Object Enabler

Martin Stewart
AEC Support Specialist