Failed to fetch dep11/by-hash File has unexpected size

[jjakob]

With 50appstream in apt.conf.d, the following errors occur (example):

Err:10 http://apt-cacher.foo.org:3141/ubuntu bionic/main DEP-11 48x48 Icons
  File has unexpected size (162304 != 118319). Mirror sync in progress? [IP: 10.1.2.3 3141]
  Hashes of expected file:
   - Filesize:118319 [weak]
   - SHA256:72a445c7d354db04d1ffca6f52f7ab66953a3fa889bb66bf6d3210d3ab100f14
   - SHA1:176f580a423a2fcf3e257f0d9a00713e05545a35 [weak]
   - MD5Sum:bdfd4c6a690b2f9d0790443956b327fd [weak]
  Release file created at: Thu, 26 Apr 2018 23:37:48 +0000

Going directly to the mirror set in the config toml, everything works fine.

Full outputs:
apt-get-update.notok.txt
go-apt-cacher.si-ubuntu.notok.log
apt-get-update.ok.txt

Configs:
go-apt-cacher.toml.txt
50appstream.txt

Tested on Ubuntu 18.04 desktop live iso. (same on 16.04/18.04 desktop if installed with 50appstream)