The most probable cause for this is that you have accidentally built icon data into a grid cell in the 24×24 column.
To correct this, click on each cell in the 24×24 column and select “Clear Resource” from the Resouce Menu at the top of the filter. Once this is done, re-save the file and see if the problem has been corrected.
There are more tips for getting problem icons to display correctly in the IconBuilder for Windows User’s Guide that comes in the download.
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