Hi Michael, sorry for the delay but I had to get some sleep. Just to summarize:
The JSON syntax issue with the "scalar" element I got on the Joomla 3.8.3 system. In this case I did not get the white screen but when I tried to create the icon the JSON syntax error occurred. When I fixed the database, the ICON was there and worked. I deleted it and created a new one without a problem. I sibsequently updated the system to Joomla 3.8.5. Still no problem.
The white screen still occurs on the other Joomla 3.8.5 system, where I have never been able to create an Icon. I checked the database after the white screen and there is no entry for the favicon plugin etc. So the error occurs before the database is written.
You will probably not be able to recreate the JSON problem unless you have a Joomla 3.8.3 system somewhere.
Let me know if there is anything else I can provide to make it clearer...Tx