Subdirectory in Nextcloud is undeletable

When I started using Nextcloud, I put a folder featuring artwork that I had made up to that point, however, I ran into storage issues and tried deleting it, only to get this error:
I went into the folder and was able to manually delete all but one subdirectory, which refused to budge. The subdirectory’s contents were all undeletable, unmodifiable, and couldn’t be added to, though overall small enough that I could continue using Nextcloud.
What I think happened is that folder decided to become read-only even though I hadn’t specified it at all.
It’d be nice if I could remove it, or for an admin to delete it.