I having the same issue on the settings page when I use custom labels for thumbnail sizes. Otherwise, submiting changes with default values for labels works as excepted.
I found that the 151th line error was caused due to a serialized value ($sizesSettings) used as an array but whithout being correctly unserialized (in my case a double nested serial value was given).
I fix this adding temporaly a second "unserialize()" call after line 133 of "manual-image-crop/lib/ManualImageCropSettingsPage.php" to be able to restore and save my settings:
$sizesSettings = unserialize($this->options['sizes_settings']);
$sizesSettings = unserialize($sizesSettings);
By the way, thanks a lot Tomasz for this great addon!