Using my powers of logic, this guy gets an 'Illegal Operation' error when using the tileset.

So, do you get the error when opening JCS and loading the tileset or when opening a level using the tileset? If the error occurs when opening the level, then the level is probably corrupted (The level has bad data). If the error occurs when the tileset is opened in JCS, the tileset is probably corrupted.

Prognosis: Re-download the tileset/level from another source.

-ThO