Hi Aki,
I failed to disclose that the described problem occurs on iOS 13.0
beta.
After trying again and again, it appears that a bug in iOS 13.0 beta
is the likely culprit. I am reading on Reddit that there is some bug
in iOS with certificate trust...
https://www.reddit.com/r/signal/comments/c2q6c6/anyone_using_signal_in_ios_13_beta_iphone/