I'm posting this for the benefit of WebTerminal users who have upgraded to the recently-released IRIS 2024.2 -- (Build 247U) Tue Jul 16 2024 09:52:30 EDT -- or are considering doing so.
That version of 2024.2 contains a change (DP-432503) which requires that the user under which the Web Gateway initially connects to IRIS (usually CSPSystem) must have READ permission on the database hosting the dispatch class of the REST web application.