Debian/Ubuntu repository: repo-keys not working

I installed SyncThing on a Ubuntu 12.04 server. It performs well but I had trouble with the repo-keys. This is what I did following the instructions from the repo page

# Add the release PGP keys:
curl -s | sudo apt-key add -
# Add the "release" channel to your APT sources:
sudo sh -c 'echo deb syncthing release > /etc/apt/sources.list.d/syncthing-release.list'
# Update and install syncthing:
sudo apt-get install syncthing

The following warning says that the package cannot be authenticated. Over-riding this issue, the installation proceeds otherwise fine.

WARNUNG: Die folgenden Pakete können nicht authentifiziert werden!
Diese Pakete ohne Überprüfung installieren [j/N]? j
Hole:1 syncthing/release syncthing amd64 0.11.7 [3'169 kB]
Entpacken von syncthing (aus .../syncthing_0.11.7_amd64.deb) ...
syncthing (0.11.7) wird eingerichtet ...

The key is registered:

sudo apt-key list
pub   2048R/00654A3E 2014-12-29
uid                  Syncthing Release Management 
sub   2048R/F614F575 2014-12-29
sub   4096R/37C86D31 2015-05-11 [expires: 2025-05-08]

Has anybody else encountered the same issue?

I did some googling the previous time this came up and it seems dpkg has a bunch of bugs that prevents it from successfully checking the signature in some cases;

OK, thanks for your instant reply! I had not recognized that the thread about Raspbian treats the same issue. Am just gonna let it stand as is. Many thanks for this great piece of software!