During development, when trying to save an Interactive Grid on either a public page, or when the application has no authentication scheme, you get this error (seen in Apex 19.1):

1 error has occurred
Ajax call returned server error ORA-20987: APEX - An internal error has occurred while processing the Interactive Grid request.
- Not authenticated. for .

In Apex 19.2 the error is a bit different (you can get more info by using the link in the Javascript console of your browser):

An internal error has occurred while processing the Interactive Grid request.

Apparently this is a known bug (Oracle bug #25177537). For more info and a workaround take a look here: https://www.databaseusers.com/article/5990721/APEX+5.1+Cannot+Save+a+non-editable+Interactive+Grid+when+page+is+public