I just tried to send another message via contact, and again, the Captcha verification failed, after it had indicated success.
And of course now, due to my attempts, my “daily conversion limit” has been reached, so I cannot load any files.
I don’t see a way to upload / send you the original stl either on your site, or here in the thread. 
Here are two parts of the session console output, unfortunately it wouldn’t allow me to copy the job ID, and I didn’t think at the time to do a screenshot.
…because it said it had succeeded. It wasn’t until I went to open the file and noticed it was only 39 bytes and no geometry.
Maybe you can find it by the time stamps?
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Found it, and it was neither the conversion nor your file. The repair actually succeeded: 9,974 triangles, 0.5 MB, and uploaded the result fine.
The 39 bytes were the security related error message. Downloads were behind a captcha session that expires after 10 minutes, and your conversion took 7m41s, so by the time you clicked download the session was gone. The server sent back {"error":"Captcha verification failed"}, which is exactly 39 bytes, and your browser saved that as the .stl. Two other people lost files the same way that day.
Fixed, downloads are no longer gated. You can try the conversion again.
The contact form was fixed and supported the attachments. When you tried again, your browser was using the cached old version. I have fixed the stale cached html living in the browser as well.
If you make a free account you can claim the free Basic upgrade, 20 conversions a day and 100 MB per file instead of 5 and 25 MB.
Thanks for sharing the details. Three users were silently losing their output and I had no idea until your screenshots.
