SDP MSP 9405 change Account, Site and Requester via REST API

SDP MSP 9405 change Account, Site and Requester via REST API

Hello. I'm trying to change the account, site and requester of a request by using the REST API in SDP MSP 9405. This is the JSON data I send:
'{
"operation": {
"details": {
"level": "Einfach",
"priority": "Niedrig",
"Abrechenbar": "Nein",
"category": "Allgemein",
"site": "Intern",
"account": "our Company name including a & symbol",
"requester": "Dummy User company name"
}
}
}'

FYI "Abrechenbar" is a custom field. If I leave site, account and requester out the API call is working successful, so the error is somehow related to site/account/requester. The API url i'm calling is " https://our-url.tld:443/sdpapi/request/<TICKET ID>". The answer I'm getting is:

{"operation":{"result":{"message":"Error when editing request details - Exception while updating Workorder. For input string: \"null\"","status":"Failed"}}}

I used an API call to "/sdpapi/request/<Ticket ID>/details" to get all fields and possible values. For "account" it states "type : Not Editable", which confuses me. In the in-app API documentation "Admin" - "General" - "API" it lists under "Edit Request" the variables "subject", "description", "requester", "site", "account" available for using.

Also the API documentation and examples are really bad. A while ago I could access https://intranet.wiki.zoho.com/MSP-SDP/REST-API-for-native-app.html which was slightly better, but access is now blocked.