See the thread labeled "Map Editor" started by MCO. The information there indicates that range check errors that occur when adding a planet are linked to the fact that planets are named for the system, plus a roman numeral. If other planets in the system have a different naming convention, a new planet will cause a range check error as the editor tries to create a default name and can't figure out the right default.
Hope that was clear enough - I kinda lost myself in the middle of that paragraph...
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