Is the Bro 2.5.5 source code still available on-line?
The reason that you were not able to find it is probably that the directory listing broke a bit and is missing a few items - that is a very recent change, I am looking into what exactly broke there.
This is fixed. The reason that this broke is that we recently added a whole bunch of files - which led to a new code path being used (because several S3 requests were required to get the list of files), which truncated the output.
Now https://download.zeek.org shows all available files again - and this error should not reoccur.
Note that this only affected the directory listings - the files always were there, and available for download with the direct filename.