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.