'apt-get update' gets stuck for hours at "100% [Working]" for specific PPA

When I run sudo apt-get update on Ubuntu 15.04, I get the following (shortened) output:

$ LANG=C sudo apt-get update
[sudo] password for ipc-admin: 
Ign http://ftp.uni-stuttgart.de vivid InRelease
Ign http://ftp.uni-stuttgart.de vivid-updates InRelease
.
.
.
Hit http://ftp.uni-stuttgart.de vivid-proposed/universe Translation-en
Hit http://ftp.uni-stuttgart.de vivid-proposed/universe Translation-de
100% [Working]

Then it gets stuck and won't continue until I interrupt it with Ctrl+C.

When I do sudo aptitude update instead, it also hangs at the same position.

After deleting the cache with sudo rm -rf /var/lib/apt/lists and running sudo apt-get update again, I still get the same results.

However, it seems to have successfully updated the package index files after running sudo apt-get update and interrupting it when it gets stuck, as I still can run apt-get upgrade and receive new updates, or apt-get install. It just does not finish the apt-get update task...

A reinstall of apt with sudo aptitude reinstall apt and a reboot afterwards did not change my situation either.

What might be the problem here and how can I fix it?

PS: The full output of apt-get update recorded with script that also shows those dynamic lines may be found on pastebin.

UPDATE

After letting it run for hours, it actually finishes! The repositories that cause the problem are various https://repo.tox.im lines. Here what appears after loooong times of waiting between each line:

Ign https://repo.tox.im nightly InRelease
Ign https://repo.tox.im nightly Release.gpg
Ign https://repo.tox.im nightly Release
Err https://repo.tox.im nightly/main i386 Packages
  Connection timed out after 120000 milliseconds
Ign https://repo.tox.im nightly/main Translation-en
Ign https://repo.tox.im nightly/main Translation-de
Fetched 69,8 kB in 36min 0s (32 B/s)
W: Failed to fetch https://repo.tox.im/dists/nightly/main/binary-i386/Packages  Connection timed out after 120000 milliseconds

E: Some index files failed to download. They have been ignored, or old ones used instead.

After this, the process terminates correctly without having to interrupt it. So what can be the problem with that repository and how may I fix it?

After having waited for the command to complete for hours, I saw that it hangs on all tox repositories.

I checked their main page and found out, that the whole infrastructure including repositories, wiki and main site moved to a different domain. That's why the old URL is not alive any more and why apt-get update is timing out.

On their new wiki, they gave the following (slightly modified) script to install the actualized repository:

#!/bin/bash
sudo apt-key del 0C2E03A0
sudo sh -c 'echo "deb https://pkg.tox.chat/debian nightly main" > /etc/apt/sources.list.d/tox.list'
wget -qO - https://pkg.tox.chat/debian/pub.gpg.key | sudo apt-key add -
sudo apt-get install apt-transport-https
sudo apt-get update
echo "Tox Repository Installed. You can now install qtox"

I executed those lines and now all problems with apt-get update are gone!

Unfortunately, the new repository does only contain 64bit builds of qTox yet, so I will have to live with that package being hold back for a while, until they also compile it for 32bit machines!

WARNING: If you have 32bit qTox installed, do not install the new repository! It contains a 32bit qtox package with higher version number than the last one available, so apt-get upgrade will install it and replace your current qTox installation with that one, as soon as you run updates. But it is not a 32bit package, it just looks like one! Inside it contains 64bit executables!

For more information and a script to verify the package's true architecture without having to install it, see my Q&A here!

The last repository update fixed this and qTox is now available for both architectures again! :)

I recognized this problem from time to time and from amchine to machine. I got rid of it by simply changing the to main mirror server - called in the german version you are using "Haupt-Server"

Since I changed away from "German Servers" to "Main Server" everything is fine...