Call Web Service action returning Internal Server Error

error 500 usually means the request is not correct or some objects referenced in the request are not valid (eg. user, group,...)

you do not show how did you set up the most important part of the request body, so hard to say what might be wrong with your request.

for me eg. following request works without any problems

(sensitive data redacted)

‌ add user to group‌ ‌ ‌

I do not have experience with excel webservices, but it sounds to me suspicious if you reference excel file to open with your local path...

How does web service would access it???

I'd say it should provided as an URL/UNC path or a path relative to excel web service root.

I've found eg, an example - https://msdn.microsoft.com/en-us/library/microsoft.office.excel.server.webservices.excelservice.openworkbook(v=office.14…