Actually, had to do this on an old Owncloud instance where I was not able to export the calendar data to be imported into a new NextCloud instance.
On Old Server
Log into the database for OwnCloud
Find the calendarid of the user in question (don't remember query)
exit database
execute following command<code bash>mysql owncloud -e 'select calendardata from oc_clndr_objects where calendarid = 32' > calendar32.ics
copy to workstation
On Workstation
Open with geany
search and replace all '\n' with nothing. If you feel brave, replace with newlines
convert to Linux line endings
save
Open Thunderbird
Go to calendar
File | Open, find ics file
Export Calendar
Open Nextcloud web interface
Calendars
Import