Referencing in Maya file breaks substance materials

[mattock]

Hi, I am using substance source materials in Maya 2020 using the latest 2.1.7 plugin, with Vray next.
All looks great, but when I then reference in this file into my scene ALL substance materials  break. 

They just show black and get errors such as cant create thumbnail etc
Sometimes they work on first reference in, but as soon as I save the file they disappear

Any idea why substance source materials wont work when in a referenced file?
And how to get it to work, much appreciated thank you.

Also the plugin seems to cause a lot of lag and slow down in Maya sporadically.
Opening a simple file can take ages.  If I try and open the new file that I have referenced my file into with the substance source textures now broken, it gets even worse.

Sometimes over ten minutes, and sometimes not at all, and I have to force quit maya
I have decent workstation, HPz840,  128ram and Nvidia P5000 Graphics card.

Any help greatly appreciated, as I've designed this character now, I don't want to have to abandon all the material set up.
Do I need to bake out the textures and rebuild a vray material perhaps?... which seems crazy to have to do that
Many thanks

[Galen Helfter]

It has to render all the sbsars on load for now. I'll look at seeing if we can add a caching system to speed up load times in the future.

For referenced scenes, I'm not entirely sure, but I'll take a look at it. It may be that it doesn't import correctly. I'll take a look at see what I find.