I have not been able to determine the root cause of this problem but thought I should report it in case someone else is experiencing this.
I don't think this was happening in the past but it might have been and I didn't notice.
On my Subsonic 6.0 which sites behind an apache reverse proxy, when I try to fetch the file :
https://www.example.com/icons/subsonic_logo.png
40960 bytes of the 43476 byte file are fetched and then the connection just hangs and waits for 669 seconds and then times out.
When I fetch the file from localhost it downloads fine.
If I change the contents of the subsonic_logo.png file to something else, a bigger image, it downloads fine.
If I copy subsonic_logo.png to another filename, the other file also exhibits the same hanging on download
Other static files in subsonic download just fine
In looking at what was going in wireshark, the TCP connection does a TCP Keep-Alive every 10 seconds for 60 seconds then does nothing until 665 seconds then does 4 Keep-Alives and finally does an RST
My final workaround was to load the image into GIMP, resave it and in doing so change it's contents ever so slightly by re-running the png lossless compression and upload it back. The file no downloads just fine.
No idea what's going on here. The number 40960 is interesting. This could have been happening in the past where this occurred, my browser cached the incomplete file (which you can visually see is missing the bottom of the submarine) and from then on served up the cached version and I happened to do a shift-refresh today forcing a re-download. Don't know.
Anonymous
I've found another example of this happening so it's not just the one file I mention above that's affected.
One piece of cover art just took 671 seconds before it timedout and only partially loaded.
URL was of the form of https://example.com/coverArt.view?size=160&id=137567&auth=123123123
In this case the image loads 7755 bytes of the 8712 total bytes before stalling.
This is very weird.