5.9-RC2 vtkOSPRayMaterialLibrary: JSON parsing error

I have 5.9-RC2 compiled on my desktop, and on 3 different Cray servers. No problem with the first two servers which have well-tuned installs. The 3rd Cray is a new beast. Batch-mode works fine (so far). But when connecting my client - just like I do with the other two Crays - I get this error:

vtkOSPRayMaterialLibrar:280 ERR| vtkOSPRayMaterialLibrary (0x560dcf41b340): JSON parsing error: * Line 1, Column 1
Syntax error: value, object or array expected.
( 17.891s) [paraview ]vtkSIProxyDefinitionMan:525 ERR| vtkSIProxyDefinitionManager (0x560dd27bcd10): No proxy that matches: group=writers and proxy=PPLYWriter were found.
( 17.891s) [paraview ] vtkSMProxy.cxx:1699 ERR| vtkSMDataExtractWriterProxy (0x560dbd72e620): Failed to create subproxy: PPLYWriter
( 18.641s) [paraview ]vtkSIProxyDefinitionMan:525 ERR| vtkSIProxyDefinitionManager (0x560dd27bcd10): No proxy that matches: group=writers and proxy=PPLYWriter were found.
( 18.642s) [paraview ] vtkSMProxy.cxx:1699 ERR| vtkSMDataExtractWriterProxy (0x560dcf44d320): Failed to create subproxy: PPLYWriter

the end effect is that part of my menus have disappeared. For example, I cannot create a Contour of a Wavelet. I don’t have any Contour available (no icon) and it is also disabled in the Filters menu.

Facts: I do not have raytracing compiled on the server side. So it should not try to load ospray_mats.json on the server side (If it needs to.Am not sure). Verbose output tells me that the client is loading it OK, but then the error…

Do you have tips on debugging this issue? TIA