If you changed the parent forums at all while cleaning up forum home, in 3.0.14 this was an issue. This could also occur if the datastore is missing or incorrect when you try to save a sub-forum. It could have been a problem with the multi-version upgrade from 2.5.7 all the way to 3.0.14, and if so, is pretty much impossible now to determine which step may have been problematic.
To get your content back, go to AdminCP > VaultWiki > Wiki URL Manager, and click Rebuild Page URLs.