Thanks.
I reproduce your problem and I will update the HikaSerial package as soon as possible.
Before that, you can fix the attach serial plugin manually.
Please edit the file "plugins/hikaserial/attachserial/attachserial_configuration.php" and replace the code
$fonts = JFolder::files(HIKASERIAL_MEDIA . 'ttf' . DS, '.ttf');
$default_font = @$this->element->plugin_params->default_font;
By:
$fonts = JFolder::files(HIKASERIAL_MEDIA . 'ttf' . DS, '.ttf');
$fonts = array_combine($fonts, $fonts);
$default_font = @$this->element->plugin_params->default_font;
You will have to re-save your attach serial configuration with your right default font.
Regards,