Sorry.
One of the add-ons you have configured for updates apparently doesn't contain a version number.
The only real explanation for this is if you aren't using a Resource Manager URL or the add-on has been deleted...
Can you post a screenshot of the add-ons you have configured for updates?
Also, to help identify the specific add-on you could edit a file and run the check again.
In library/AddOnInstaller/Model/AddOn.php
Search for:
PHP:
$request = $client->request('GET');
Replace with:
PHP:
$request = $client->request('GET');
Zend_Debug::dump($addOn['title']);
When you get the error, there should be a plain text dump of a list of add-on names at the top of the page. The very last one is the one throwing this error.
I've written the links to the resource manager, not external ones or deleted resources.
Here are all the screenshots:




This is the output:
string(28) "Add Poll to Existing Threads"
string(24) "Add-on Install & Upgrade"
string(34) "AutoComplete Combo Box by Waindigo"
string(14) "Avatar Gallery"
string(8) "AzuCloud"
string(20) "Conversation Manager"
string(21) "Custom BBCode Manager"
string(13) "Developer Kit"
string(11) "Double Post"
string(28) "Enable Debug From Admin Home"
string(16) "First Click Free"
string(34) "FreddysHouse Tools: Signature Once"
string(12) "Live Updates"
string(25) "Login As User by Waindigo"
string(28) "New User Notification System"
string(23) "No Downtime by Waindigo"
string(24) "Post As User by Waindigo"
string(20) "Prefix Forum Listing"
string(18) "Profile View Count"
string(17) "Quote de XenFacil"
string(23) "Registration Form Timer"
string(34) "Remove Title From Conversation URL"
string(13) "Route Changer"
string(6) "Social"
string(20) "Sub-nodes in columns"