login: Unhandled exception while sending code

[V02460]

When trying to login via the bridge bot chat flow, i.e., entering login followed by a phone number and the received SMS verification code, the bot answers with:

Unhandled exception while sending code. Check console for more details.

The bridge logs show these two errors:

mau.commands: Error sending phone code                                                                                                                           
Traceback (most recent call last):                                                                                                                               
  File "/usr/lib/python3.9/site-packages/mautrix_telegram/commands/telegram/auth.py", line 280, in enter_code                                                    
    await _sign_in(evt, code=evt.args[0])                                                                                                                        
  File "/usr/lib/python3.9/site-packages/mautrix_telegram/commands/telegram/auth.py", line 313, in _sign_in                                                      
    user = await login_as.client.sign_in(**sign_in_info)                                                                                                         
  File "/usr/lib/python3.9/site-packages/telethon/client/auth.py", line 372, in sign_in

mau.commands: Error sending phone code
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/mautrix_telegram/commands/telegram/auth.py", line 280, in enter_code
    await _sign_in(evt, code=evt.args[0])
  File "/usr/lib/python3.9/site-packages/mautrix_telegram/commands/telegram/auth.py", line 313, in _sign_in
    user = await login_as.client.sign_in(**sign_in_info)
  File "/usr/lib/python3.9/site-packages/telethon/client/auth.py", line 372, in sign_in
    raise errors.PhoneNumberUnoccupiedError(request=request)
telethon.errors.rpcerrorlist.PhoneNumberUnoccupiedError: The phone number is not yet being used (caused by SignInRequest)

The Telegram account in question was active on the bridge before.