Had the same problem. This helped:
WordPress Sociable Fix
From the page:
I’ve spotted a bug in the current version of the plugin (version 2.0) which prevents people from bookmarking your posts to StumbleUpon. If anyone tries to bookmark a page to StumbleUpon they will be taken to an invalid URL. The problem is with the following piece of code in sociable.php
‘StumbleUpon’ => Array(
‘favicon’ => ‘stumbleupon.png’,
‘url’ => ‘https://www.stumbleupon.com/url/PERMALINK‘,
To fix the problem, change the above code to this:
‘StumbleUpon’ => Array(
‘favicon’ => ‘stumbleupon.png’,
‘url’ => ‘https://www.stumbleupon.com/submit/?url=PERMALINK&title=TITLE’,
You may want to temporarily disable the plugin whilst you make this change.
