Processing fails with "Error in input stream" message

This happens after I click on `begin` and I get the loading page. The page behaves normally for a while, and then this appears:

The logs say this:

```
medieval manuscripts preset: loaded text/music + stave detectors
CH-Fco Ms. 2_006rjpg.jpg: layer separation done (110 detection(s))
running Kraken segmentation + HTR across 1 folio(s) (Cantus-aligned mode, source 123672)...
1 image(s) staged for folios 006r–006r
running Kraken segmentation + HTR across 1 folio(s) (Cantus-aligned mode, source 123672)...
[INFO] Folio 1/1: 006r
[INFO] Applied mothra mask: 99 text bboxes, padding=15px
[INFO] Initialized collection 'None' with 1 pages
[INFO] Stage 1: Kraken line segmentation
[INFO] No segmentation model given. Loading default model.
[INFO] Segmenting
error: Error in input stream
```

This happens approx. 1 out of 3 times that I try running a folio. It's happened for multiple different folios, with and without cantus ID (I think). Usually, if I hard refresh the page, my next run works, but after that all bets are off. I haven't yet figured out how to reproduce it...