https://github.com/wchill/patcheddit/releases/tag/v2.0.1
Based on some testing that was between me, the devs of Apollo Reborn, and the Continuum/Slide dev (at different points in time), Reddit is starting to restrict API access by route. 403 errors related to tapping on "X more replies" is caused by this - RedReader doesn't use the /api/morechildren API route, so presumably Reddit decided to block that route because it wouldn't affect legitimate RedReader users.
The same restrictions apply to /api/info and /by_id, though I think these won't affect users in general.
I've fixed this issue in Boost by hijacking the /api/morechildren calls and replacing them with calls to /comments/<post id>/comment/<comment id>.json which is what RedReader uses, and will extend this fix plus the gallery fix to all other apps supported by Patcheddit soon.
This does seem to be dependent on the client ID, but I would not recommend switching back to your own unless you want your app to stop working on January 12, 2027. Reddit is blocking all non-approved client IDs starting on that date.