I've been experimenting with the new datapacks in the 1.13 snapshots, and I think I've come across a bug.
When reloading functions (as you could in previous updates), you get the error:
As far as I can tell, this error only happens for functions - not advancements, structures, or other datapack files. This does, however, happen whether the functions folder is empty, has .mcfunction files in it, or has subfolders with .mcfunction files.
If the functions are already present before entering the world, the world loading process gets stuck in the 'Loading world' stage.
As the bug might not be apparent on first load, exiting and re-loading the world (or using the /reload command) might cause it as well.
I've attached a zipped version of the datapack I'm using. Note that I'm using the unzipped version in order to get the error. I've also attached the error note from the output log, in case that helps.