Managed to fix the issue by removing these lines:
<?php foreach($blogs as $blog): ?>
<?php $blog_details = get_blog_details($blog->blog_id); ?>
<option value=”<?php echo $blog[‘blog_id’] ?>”><?php echo $blog_details->blogname; ?></option>
<?php endforeach; ?>
Now the priorities can be set. This site has a bit special multisite setup so maybe that was causing the issue.