I recommended you to do 3 things in #14, now do the other two things. The _rspamd user is added to the amavis group when you run the ispconfig update with reconfigure services = yes (which is also part of the Rspamd instructions), so I wonder why this user was not added to the group or if you did not do the step from the guide, which would explain why signing still fails as this step does other required configs as well.
Then check again that the _rspamd user is a member of the amavis group and restart Rspamd after you changed that.
