Tails-Upgrader "cannot choose a download server"

there were multiple reports about tails-upgrader crashing with this error :

Error message stating "cannot chose a download server" "This should not happen. Please report a bug"

The following is copied from terminal:

Prototype mismatch: sub Tails::MirrorPool::assert ($;$) vs none at /usr/share/perl5/Tails/MirrorPool.pm line 28.
Prototype mismatch: sub Tails::IUK::Frontend::assert ($;$) vs none at /usr/share/perl5/Tails/IUK/Frontend.pm line 42.
This version of Tails is outdated, and may have security issues.
Upgrade available
<b>You should upgrade to Tails 2.5.</b>

For more information about this new version, go to https://tails.boum.org/news/version_2.5/

It is recommended to close all the open applications during the upgrade.
Downloading the upgrade might take a long time, from several minutes to a few hours.
The networking will be disabled after downloading the upgrade.

Download size: 324.66MiB

Do you want to upgrade now?: Upgrade now / Upgrade later

** (zenity:5402): WARNING **: Couldn't register with accessibility bus: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
Downloading the upgrade to Tails 2.5...

undefined:183
    },
    ^
SyntaxError: Unexpected token }
    at Object.parse (native)
    at Object.<anonymous> (/usr/local/bin/tails-transform-mirror-url:22:8)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
Error while choosing a download server
<b>Could not choose a download server.</b>

This should not happen. Please report a bug.

** (zenity:5409): WARNING **: Couldn't register with accessibility bus: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.
Error while choosing a download server
<b>Could not choose a download server.</b>

This should not happen. Please report a bug.
"tails-transform-mirror-url" unexpectedly returned exit value 8 at /usr/share/perl5/Tails/MirrorPool.pm line 82.
 at /usr/share/perl5/Tails/IUK/Frontend.pm line 593.

Feature Branch: bugfix/11735-mirror-pool-size, mirror-pool-dispatcher:bugfix/11735-mirror-pool-size

Related issues

• Blocks #12405 (closed)

Original created by @goupille on 11735 (Redmine)