Hi Karl,
try replacing the line 37 of "/application/views/timeslot/object_timeslots.php" with this code:
<?php if ($timeslot->getUser() instanceof User) { ?>
<td style="padding-right:10px"><b><a class="internalLink" href="<?php echo $timeslot->getUser()->getCardUrl()?>" title=" <?php echo lang('user card of', clean($timeslot->getUser()->getDisplayName())) ?>"><?php echo clean($timeslot->getUser()->getDisplayName()) ?></a></b></td>
<?php } else {?>
<td style="padding-right:10px"><b><?php echo lang("n/a") ?></b></td>
<?php } ?>