I am having the exact same issue with the updateMember function. Although I do see that the wixUsersBackend . updateUserFields is a depricated function.
I get the following error when trying to run the updateMember function.
{
"message": "Error: Unable to handle the request. Contact the site administrator or view site monitoring logs for more information.",
"name": "Error",
"stack": "Error: Unable to handle the request. Contact the site administrator or view site monitoring logs for more information.",
"_elementoryError": true
}
Not sure if you are seeing the same thing or not.