My last question is in a way solved
pluginplugin-siteorigin-csstechnical · By MoniqueOostdam · 8 years ago · Reply by Alex S 8 years ago
While I was reading the other questions, I discovered that I have a plugin conflict too. Also with the plugin Woo Product Multi Layout. I have deactivated that plugin and now I can work with Custom CSS again. Thanks, Monique