I suppose you tried to use the sharing pane on Welcome.md and removed the owner part, locking yourself out of the file leaving only Read for everybody.
As server admin, I renamed Welcome.acl to Welcome.acl.txt so you can see the acl content.
Everything should be working now.
This is a known problem. Always check to keep at least ReadWrite access.
You can ask for support and report issue on GitHub - solid/solidcommunity.net: Operational issue tracking for solidcommunity.net