M-q does not fill quoted text anymore

[Mathias Dahl]

Recently, M-q has stopped working for me when quoting messages in
Mail-mode. It does no longer prefix each line with the quoting
character used (normally ">").

When writing this message, in Gnus, it works similar, but here the
auto fill does the right thing even though M-q does not work.

Any idea where to look to fix this? I am sure I installed "something"
recently that have messed this up.