Opening of emails are captured by using a tracker images. However due to a bug the data-analysis did not work properly, so the statistics did not show the correct data. To fix the bug please open
/mailhive/common/classes/clicktracker.php
find line 226 and change it to
if (substr_count($line, '|') < <span style="color: #000000;"><strong>2</strong></span>) {
(so replace 3 with 2) - done.
This bugfix will be part of the next release.