Okay, concerning the rule thing this got another flavor after some more trying: When I recreated the FO, still wrong rules were executed. I re-checked the ruleset customizing and found everything ok, clicking "Save" just told me "Data already saved". However, after I changed the name of an activity, saved, re-changed and saved again, the event message could be posted and the correct rules were applied!
Seems like there are situations where changes made to the ruleset customizing are not persisted properly upon saving...