When selecting world to upload, message "Preparing your world" first appears, and then the "back" button appears, never successfully uploads the selected world.

The datapack is currently valid.

In the datapack, all advancements, functions, loot tables, recipes, and tags are valid, tested, and work as intended.

In fact this same world is currently successfully running on the very same Realm world.

I even tried downloading the current backup, and then simply re-uploading it to test, and it still won't successfully upload, so it even rejects the very world it is currently successfully running.

I am attempting to update a few minor details (to correct actionbar message competition in various mcfunctions), and am trying to re-upload the updated world.

When I look at the downloaded realms-upload.log, the most recent entries there are curious:

Rejected world/datapacks/GatPack/data/minecraft/recipes/.DS_Store: Unsupported path
Rejected world/datapacks/GatPack/data/minecraft/structures/.DS_Store: Unsupported path
Rejected world/datapacks/GatPack/data/minecraft/tags/.DS_Store: Unsupported path
Rejected world/datapacks/GatPack/data/minecraft/tags/functions/.DS_Store: Unsupported path
Rejected world/icon.png: Unsupported path
Rejected world/realms-upload.log: Unsupported path

It would appear that it is even rejecting it's own .log and icon files as having an unsupported paths!

The .DS_Store: that is says has Unsupported path must be on Realm-side. No such files (not even such hidden files) exists in my world folder/file structure.

Edit to add: I notice most of the "Unsupported paths" relate to world/datapacks/namespace/data/minecraft, so perhaps Realms is trying to prevent users from altering vanilla minecraft functions on Realms? However, users must have access to at least world/datapacks/namespace/data/minecraft/tags/functions/tick.json in order to add the datapack functions that must run every tick.