Hi William,
We have been seeing the same issue, among others while upgrading out non-production environment to 10.9.1. Our issue has been with trying to create, change or save domains on Hosted Layers like you mentioned. The only difference for us is that our values were string code/description values and being assigned to a string field.
Dev Tools in the browser reports the following error for us. This error is reported no matter the type of field/domain/change we make:
{
"code": 500,
"message": "JSONObject[\"description\"] is not a string.",
"details": []
}
I have tested this on hosted service I published or by using one of ESRI's template feature services as well as hosted services I have created.
For the screen shot below I simply added "Now" to the end of the "Shelter in Place" domain in the Status List of domains. This domain was created automatically with the Check In template in the Create a Feature Layer menu.
We have not yet created a support ticket for the issue yet as we want to get a few more tasks done in our upgrade completed before we create tickets for UI issues. Interested to see what you learn. I will follow up once we talk to Support.
Will