With the beginning of WeeChat 2.4 (released 2019-02-17) a space is no longer automatically added when you complete a nick at the beginning of command line.
This leaves you with
Hund:_ and not
Hund: _. If you want the old behaviour back like me, the solution is simple. You just need to change one simple setting:
/set weechat.completion.nick_completer ": "
Don’t forge to save the changes with