You are right. The configuration which lead to the errors was this one.
The ping count I did not change (but I tested it also with and without - same result).
Such a dependency for a watchdog would not be exactly helpful, when e.g. logind hangs (as well), but who knows. systemd added it as dependency for quite a bunch of features with Bookworm (the related systemd version).
To rule out the systemd unit: when stipping the service, and running watchdog from console with the arguments/options from /etc/default/watchdog, does it work? Ah hence just watchdog ping`.
Ahh wait a second, this cannot work. The watchdog_options are added as arguments just like that according to the systemd unit, and ping is no valid argument @trendy. There is no command-line argument to define the ping method. Instead one can set it e.g. to -c /etc/watchdog.conf to define the config file. That however is the default. So best to keep the watchdog_options variable commented in /etc/default/watchdog.
The command to test one console is hence reallt just watchdog, which then uses mentioned config file, just like the systemd service should do by default.