As a wild guess try using  instead of "", as in :
CUSTOM CONNECT TO [Provider=QvRestConnector.exe;
Also are you using a dollar expansion of a variable assigned a value using LET ?
- If so then make sure you use single quotes and not double quotes
Another also is to maybe look into disabling Standard Mode Disabling standard mode ‒ Qlik Sense
- I have had to do that recently to get an API url to work - But do beware the security implications of this.