YouTrack change Assignee with the Host API

I have this code that should change the issues of the assignee to another assignee that I selected before.

const changeIssues = async (oldAssignee: User, changeAssignee: User) => {
        setchangeAssigning(true);
        try {
            for (const issues of userIssues) {
                const response = await host.fetchYouTrack(`issues/${issues.id}`, {
                    method: 'POST',
                    headers: {
                        'Accept': 'application/json',
                        'Content-Type': 'application/json',
                    },
                    body: JSON.stringify({
                        customFields: [
                            {
                                name: 'Assignee',
                                $type: 'SingleUserIssueCustomField',
                                value: changeAssignee ? { changeAssignee} : null
                            }
                        ]
                    }),
                }) as Response;
                const result = await response.text();
            }
        } catch (error) {
            console.error('Fehler beim Ändern des Assignees:', error);
            host.alert('Es gab einen Fehler beim Ändern des Assignees.');
        } finally {
            setchangeAssigning(false);
        }
    };

I have changed the fetchYouTrack(issues/${issue.id} to fetchYouTrack(issues/${issue.id}/customFields/Assignee

But I keep getting the error:

POST http://localhost:8080/api/issues/3-32 500 (Server Error)
Fehler beim Ändern des Assignees: 
message
: 
"Request failed."
servlet
: 
"gap-rest-servlet"
status
: 
"500"
url
: 
"/api/issues/3-32"

DevTools Logs: Logs DevTools Server Logs:

WARN - lipse.jetty.server.HttpChannel - /api/issues/3-32/ 
Caused by: javax.ws.rs.ProcessingException: Resource Java method invocation error.

What am I doing wrong?

EDIT:

When I try to change it with the curl command, it works

curl -X POST 'http://localhost:8080/api/issues/3-9?fields=customFields(id,name,value(avatarUrl,buildLink,color(id),fullName,id,isResolved,localizedName,login,minutes,name,presentation,text))' -H 'Accept: application/json' -H 'Authorization: Bearer perm:YWRtaW4=.NDYtMA==.ZffUW8VLlsysJjzbMiC0N3eWWAwILa' -H 'Content-Type: application/json' -d '{ "customFields": [ {"name": "Assignee", "$type": "SingleUserIssueCustomField", "value": {"login": "Test"}}, {"name": "Priority", "$type": "SingleEnumIssueCustomField", "value": {"name": "Major"}} ] }'