An unknown exception was thrown during indexing.

This is my first attempt to use Sourcetrail. It failed with a spectacularly uninformative error message: "An unknown exception was thrown during indexing." I was attempting to index the code from the open source repository at https://github.com/FDA/openfda. I pulled this repo into a local directory on my machine, then tried to index it using Sourcetrail. The OpenFDA code uses Python more than any other language, so I told Sourcetrail that it was a "Python source group". I also told it to look for .py, .html, .js, .sh, and .json files, since those are all included in the repo. I ran the indexing, which went on for 20+ minutes before crashing with this veritably useless error message. What does it mean?