At the possibility that it is a problem with your files, try verifying your cache integrity with Steam. See
Problem I in the Troubleshooting Guide for assistance there.
You may also want to try deleting the .msh (just the .msh. Deleting others will cause further errors) of the file in question, in the possibility it is outdated. You can find these in the static_objects folder, in the same directory as following the tree-list in the Level Editor.
For more information about troubleshooting this, you can also provide the LevelEditor.log file. To do so, place the static object down and save the map. Then open a new map and reload the one you saved. Then by closing the Level Editor, it will generate an updated LevelEditor.log file to be analyzed. It can be found by default in Documents/HPL3.
(Also, no stress, but since this is a Development question, I will be moving this to the appropriate Technical Support subforum).