The relevant part of the file is
BEGIN:VEVENT
UID:ba69ec13e93fd8365ada167183e72bba
CATEGORIES:Sonstiges
CREATED:19700101T010000
SUMMARY:Kaffeetreff
LOCATION:Backes
DESCRIPTION:.\n
X-ALT-DESC;FMTTYPE=text/html:<p>.</p>
DTSTAMP:20230407T114407Z
DTSTART;TZID=Europe/Berlin:20230407T160000
DTEND;TZID=Europe/Berlin:20230407T180000
SEQUENCE:0
<strong>RRULE:FREQ=WEEKLY;UNTIL=20240101T000000Z;INTERVAL=2;BYDAY=+2FR</strong>
TRANSP:OPAQUE
END:VEVENT
BEGIN:VEVENT
UID:259b23d5ab9e50cc73cc3d8dcbd71a84
CATEGORIES:Sonstiges
CREATED:20230203T082503
SUMMARY:Verschönerung und Sanierung der Weihnachtsmarktstände
DESCRIPTION:.\n
X-ALT-DESC;FMTTYPE=text/html:<p>.</p>
DTSTAMP:20230407T114407Z
DTSTART;TZID=Europe/Berlin:20230429T100000
SEQUENCE:0
TRANSP:OPAQUE
END:VEVENT
I have highlighted the line that describes the repeat pattern - this says it repeats weekly until 1 jan 2024 with an interval of 2 (i.e. every 2 weeks) on the second friday
That last piece looks strange - you should not need a BYDAY specifier for a weekly repeat.
Can you upload a screenshot of the date/time section of the event editing page in JEvents so I can try to recreate the issue