Ah, okay.
I fixed my issue. Hope that helps you, too:
I only was focussed on /etc/yunohost/apps/synapse/conf/homeserver.yaml.
After changing the bind_dn in /etc/matrix-synapse/homeserver.yaml, too (and a synapse restart) my password is recognized, again. It now looks like this:
bind_dn: "uid=synapse,ou=users,dc=yunohost,dc=org"
The only thing I still don’t understand is, why Yunohost doesn’t update that file automatically when updating the Synapse-App to 1.67.0~ynh1. …
Christian