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In Settings page of IMG export, there is "Keep temp files" option. Please turn it on, and then try to export any region.
After you get the exception, please open %temp% folder in Explorer, there should be a folder (or folders) with name(s) like ExportToIMG_... Open the most recently modified ExportToIMG folder, and take a look what files are inside it.
Normally, there should be one file with hexadecimal name and MTX extension, and a corresponding LOG file. Are they really there? What's inside the LOG file?
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