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This issue was resolved in TntMPD 2.1 r34. Under the "Help" menu, click on "Check for BETA Updates" to upgrade.
I just downloaded/installed 2.1r40 and still had this problem. I didn't want to install a beta version, but it was the only one that said it worked on Vista.
When you get this error, click the "Details" and report this issue. This will report it to the forums, along with many helpful details about this error.
2.1r40 fixed this issue for me on OSX, using the new OSX version. I used the auto-updater from I think 2.1 r32. I can now double-click on a *.tntmpd file before TntMPD is running, or use the "open file" dialog to open the file. Thanks for fixing it, at least on my installation!