Since it might help anybody with the same issue, it's indeed caused bypermission issueswhen usingroot. The script doesn't terminate quickly when executing the command as root and instead creates a piece of the core definition before failing.
So first cleanup the broken core:
bin/solr delete -c mycore
Make sure that no folder(s) linger under/var/solr/datafor yourmycorecore. paymydoctor
Next create the core as thesolruser
su -u solr -c "/opt/solr/bin/solr create_core -c mycore"