Unfortunately, these logs are quite large so they are not normally stored.
You can set the advanced options --log-level=profiling --log-file=/path/to/file and it will store the logs in that file (too late for that now…)
But I think this is perhaps related to this post:
It seems that the “500” error does not have anything to do with the files being processed, but the error is detected during the file processing, which causes a strange behaviour.