Comment 6 for bug 1750625

This is a bug in 20.04 as well. I have no Release file, this is a private, unsigned repository, only Packages.xz. On first run, apt update first works. On the second run apt update issues a HTTP GET with an If-Modified-Since header:

GET /deb/Packages.xz HTTP/1.1
Host: XXX
Cache-Control: max-age=0
If-Modified-Since: Sat, 18 Apr 2020 14:19:16 GMT
User-Agent: Debian APT-HTTP/1.3 (1.6.12)

Apache HTTPD correctly responds that there was no change:

HTTP/1.1 304 Not Modified
Date: Sun, 19 Apr 2020 20:53:02 GMT
Server: Apache/2.4.18 (Ubuntu)
ETag: "960-5a39159dd8574"

Instead of using the existing downloaded version apt update tries to download Packages.bz2 etc. and finally it displays:

root@pipa11:~# apt update
Hit:1 http://mirror.hk.leaseweb.net/ubuntu bionic InRelease
Hit:2 http://mirror.hk.leaseweb.net/ubuntu bionic-updates InRelease
Hit:3 http://mirror.hk.leaseweb.net/ubuntu bionic-backports InRelease
Ign:4 http://XXX/deb InRelease
Ign:5 http://XXX/deb Release
Hit:6 http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit:7 http://XXX/deb Packages
Ign:7 http://XXX/deb Packages
Ign:7 http://XXX/deb Packages
Ign:7 http://XXX/deb Packages
Ign:7 http://XXX/deb Packages
Ign:7 http://XXX/deb Packages
Ign:7 http://fXXX/deb Packages
Err:7 http://fXXX/deb Packages
  lzma_read: Read error (7)
Reading package lists... Done
E: Failed to fetch http://XXX/deb/Packages lzma_read: Read error (7)
E: Some index files failed to download. They have been ignored, or old ones used instead.

The exit code is 100.