I managed to recreate the problem, as you say it was not the .mtl file, it was caused by a parameter in the .properties file. The furniture could be used in the plan, but an error occurred when selecting "save". To solve the problem I deleted all instances of the imported furniture, corrected the furniture library file re-imported it again.

When I created the furniture library (with items created from Blender) I copied the .properties file from a more complex model and edited them with a text editor, leaving the multiPartModel parameter set to "true" which caused the error on save.

Using the furniture editor would have avoided that problem but I was doing things manually as I had another problem earlier with the way another program handled the .mtl file.

Thanks for your help with this.