This stumped me for a while and I was not sure what I was doing wrong. Apparently it seems if you don’t have the <configSections> element in your app config file as the first thing then you get a weird error saying "Only one <configSections> element allowed ...." (see example below). The description is misleading - you will see this error even though you only have one <configSections> element in your config file. To fix this, move the <configSections> as the first element in your app config file and all will be well.
Example error:
{"Only one <configSections> element allowed per config file and if present must be the first child of the root <configuration> element. (C:\src\MyCallApp\VOIPCall\bin\Debug\VOIPCall.vshost.exe.config line 10)"}