I have examined this problem a bit deeper. I opened the Project Info panel and there Linked Files. It shows that there are missing links to the textures used by RenderZone. I wasn't aware of this. Links to the textures used by V-Ray were all okay. I chose to unplugg all RenderZone related Extensions from the Extensions Manager. After a short testing it seems that the files open quicker also and the Material Parameter panel is more responsive. I haven't used RenderZone after V-Ray was installed. So I can leave it away.