I'm sure there could be other causes, but so far, this is what I've run into that causes the list save button to be greyed out when trying to modify the list in the browser. If you try to enter a duplicate entry, it will give you an error. However, if there is an invalid entry in the table already, it will not flag as error, just won't activate the save button.
I've seen this mostly when bringing in Excel table for domain table and Excel auto filled some values wrong.
With huge domain list, I either just overwrite the entire domain list (with a clean table) using the python API, or, export the JSON from the service, load into Excel and query for duplicated values.
R_
