However I’ve noticed that Nextcloud Calendar recognizes recurring rules if importing a .ics file.
So as a workaround one can create a recurring event on Google Calendar and then import into Nextcloud.
Or edit this template (the recurring line is RRULE):
BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART;TZID=Europe/Berlin:20190213T130000
DTEND;TZID=Europe/Berlin:20190213T161500
RRULE:FREQ=WEEKLY;WKST=SU;UNTIL=20190322T225959Z;BYDAY=MO,TU,WE,TH,FR
DTSTAMP:20190216T123909Z
CREATED:20190216T123647Z
LAST-MODIFIED:20190216T123647Z
SUMMARY:Event name...
END:VEVENT
END:VCALENDAR