As of version 1.1.1, the internal structure of Uurnog has completely changed to match that of Uurnlimited. (You can tell if there's a button on the launcher labelled "Uurnog Uurnlimited", and if the characters are blacked out by default on the save-file selection screen.) This means maps made in Uurnlimited will now work in regular Uurnog, but it also means that any custom maps made before 1.1.1 will not.
So if you ever re-download Uurnog and find that loading your custom maps gives out some kind of error, and it's not just the classic "forgot to rename map to Uurnog.uurnog" gotcha, search your hard drive to see if you have a version before 1.1.1. If you don't, you might have to contact Nifflas for an older version.