Umbraco 9.5.4 and Forms 9.5.9

We upgraded Forms to 9.5.9 and it all worked fine locally. The changes were then pushed to our Azure hosted Dev environment where it also all worked OK.

The changes were then pushed to the live Azure environment and now we get this error when trying to view any user in the Users -> Forms Security section:

/umbraco/backoffice/UmbracoForms/FormSecurity/GetByUserId?userId=user-9 500 (Internal Server Error)

Then

Possibly unhandled rejection: {"data":{"ExceptionMessage":"User not found - ID: 0","ExceptionType":null,"StackTrace":null},"status":500,"config":{"method":"GET","transformRequest":[null],"transformResponse":[null],"jsonpCallbackParam":"callback","url":"backoffice/UmbracoForms/FormSecurity/GetByUserId?userId=user-9",

We have another site which is based on the same codebase and was upgraded at the same time and that works in all environments.