Earlier quoted context omitted.
As one of the replies in that thread you linked explains, this is TLS encrypted traffic, so Telia can't really do anything to influence what happens here. They don't get to see what the traffic means, and if they change any of it then the connection aborts, which doesn't result in a 401 error it just hangs up abruptly - that's how TLS is designed to work. It is entirely possible that somebody at Signal fat-fingered a…
I could not find the reference to TLS in the replies. Generally Signal does not use TLS for their messaging system, it is instead something home brewed.
https://textsecure-service.whispersystems.org/v1/config
HTTPS is HTTP protocol spoken over a TLS encrypted channel.
When these Telia users weren't able to use the Signal Desktop software, this fetch failed, with a 401 error which is the HTTP error code for Unauthorised.