Signal no longer working on iPhone 4S

I had Signal from Signal Technology Foundation on my iPhone 4S. It worked until last week. Now I have to re-register my phone number with a one-time code and I see this message:

Verification failed. (The operation couldn't be completed SignalServiceKit.NetworkManagerError error 401)

I know the code is right because a deliberate wrong code gives a different message. The same happens whether on 3G or wifi.

I suspect that the phone is now too old for Signal because the message on the screen is truncated. Is that true?

The iOS version is 9.3.6 (13G37), which should still be compatible with Signal (since I was able to install it and run it, and did not update it at the time of the change).