The import cmdline tool doesn't display the summary (containing the list of errors), when it exists because of to many errors. Only the errors are listed, but not the successful creations/modifications/deletions.
Fixed as part of Bug #45165. *** This bug has been marked as a duplicate of bug 45165 ***
UCS@school 4.3 v6 has been released. https://docs.software-univention.de/changelog-ucsschool-4.3v6-de.html If this error occurs again, please clone this bug.