Rather than try to fight thru re-opening issue 16475, I'll restate the problem here. and provide the answer.
When running from the latex2rtf gui, get the error message "Only a single file can be processed at a time" .
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As answered in this bug report,
it comes as a result of your tex filename (or folder) having multiple words. Try renaming to a single word
So it appears the gui fails to provide quotes around the input filename. If you run latex2rtf from the command line, be sure to encase the filename in quotes, or use escapes (backslashes) to handle spaces in the filename.