Since yesterday at least, apt and aptitude update have been giving
errors such as

Err:18 https://ftp.debian.org/debian stable-updates/main Sources.diff/Index
  Need 2877 compressed bytes, but limit is 2788 and original is 2788
Err:19 https://ftp.debian.org/debian stable-updates/main amd64 Packages.diff/Index
  Need 5956 compressed bytes, but limit is 5412 and original is 5412
Err:20 https://ftp.debian.org/debian stable-updates/main Translation-en.diff/Index
  Need 4171 compressed bytes, but limit is 4096 and original is 4096
Err:21 https://ftp.debian.org/debian stable-updates/main all Contents (deb).diff/Index
  Need 512 compressed bytes, but limit is 502 and original is 502
Err:68 http://debug.mirrors.debian.org/debian-debug proposed-updates-debug/main amd64 Packages.diff/Index
  Need 26223 compressed bytes, but limit is 23168 and original is 23168

(for apt) and

https://ftp.debian.org/debian stable-updates/main Sources.diff/Index [ERROR]
 Need 2877 compressed bytes, but limit is 2788 and original is 2788
https://ftp.debian.org/debian stable-updates/main i386 Packages.diff/Index [ERRO
 Need 5536 compressed bytes, but limit is 5136 and original is 5136
https://ftp.debian.org/debian stable-updates/main amd64 Packages.diff/Index [ERR
 Need 5956 compressed bytes, but limit is 5412 and original is 5412
https://ftp.debian.org/debian stable-updates/main Translation-en.diff/Index [ERR
 Need 4171 compressed bytes, but limit is 4096 and original is 4096
https://ftp.debian.org/debian stable-updates/main all Contents (deb).diff/Index
 Need 512 compressed bytes, but limit is 502 and original is 502

(for aptitude).

Retrying the update does not give any error, but apt/aptitude
does not even try to re-download the concerned files.

This occurs again. At each "apt update":

[...]
Err:18 https://ftp.debian.org/debian stable-updates/main all Contents (deb).diff/Index
  Need 2994 compressed bytes, but limit is 2986 and original is 2986
[...]