can't remove reminder of recurring event.

opto <[email protected]> Tue, 8 Aug 2017 08:23:46 -0700 (PDT)
Newsgroups gmane.comp.mozilla.devel.calendar
Message-ID <[email protected]>
the birthdays was on Aug. 4th, but the reminder doesn't go away . I can kill it with the cross top right, but it comes back. all other buttons do nothing, the dialog stays.
TB 52.2.1  , Win 10.

any ideas? Thanks,
Klaus