I found the bug.
I was able to figure out that one specific source was causing the problem. I opened to .DTA file in Notepad and inspected the parameters for that source.
The emission rate for this source is very small, and BEEST uses scientific notation for that source. BEEST also writes the source term to the .DTA file in scientific notation.
The crash occurs if then import the .DTA file.
So I edited the data file to convert the source term from scientific notation to standard decimal notation. The error then went away.
So the bug is in the .DTA import routine; the import routine doesn't handle the import properly when a source parameter is in scientific notation.
Oh - the import dialog did indicate that all of the sources were proceessed without error.
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BEEST accepts the import until it tries to work with the data from that source. The same type mismatch error occurs if I cycle through the sources in the source control without opening grid view. As soon the control tried to load the data the type mismatch occurred.
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My first guess would be that the import routine is interpreting the scientific notation format as text data rather than numeric data, and is loading the data in BEEST as text value. When the source contol tries to load the data, the type mismatch occurs when it tries to load text into a numeric field.