Wow…
So electron improved their security features with the recent version 5, but by doing this broke tons of applications because they either need User Namespaces or an SUID executeable (to launch proper isolated subprocesses).
#Signal Desktop noticed this problem and as well and "fixed" it in the worst way possible:
https://github.com/signalapp/Signal-Desktop/commit/1ca0d821078286d5953cf0d598e6b97710f816ef
On the other hand #Riot Desktop did a proper fix, which enables an SUID bit on this binary: https://github.com/vector-im/riot-web/commit/56674ea70849b3a793fa7b862945163aa10b36b8