If I recall correctly, that means the "tmapa000.dat" files (in DK's /data/ directory) aren't properly being decompressed when they should be. So the editor is instead reading the RNC compressed versions, causing this static reading.
The editor usually decompresses these files but it may have trouble doing so if the DK directory has no write permissions. The problem is most likely that you've installed Dungeon Keeper to "C:/Program Files/" (Windows is fussy about that directory) and so this can be fixed if you move DK's directory elsewhere. There was supposed to be a write permissions error but maybe it didn't pop up.
If you like, you can check whether your tmapa000.dat files are still compressed by opening them in a Hex editor or Notepad and if the first three chars are: R N C, that means it's still a compressed file and Unearth hasn't successfully decompressed it.
The compression stuff was a big pain to deal with. There's a lot of compressed maps too, Unearth actually runs an external program from a subfolder (unearthdata\rnc-decompressor\dernc.exe) for each compressed file, since the algorithms were too difficult for me to rewrite.