SOAPv2 Not Reponding to Invoke since upgrade to 1.9.3

[WillPSZV]

I've discovered that an app I was using that connected via SOAP stopped working at the exact time I upgraded to 1.9.3 (from 1.9.0.1).

I can see the WSDL when I visit /api/v2_soap?wsdl but I can't get any response from any invokes - I can see the queries going through the DB but then the queries just sleep and timeout. There's no errors logging anywhere.

How can I debug this?

I'm running on Nginx if that makes a diff.

[WillPSZV]

Ahhah, I've found this in the PHP-fpm slow process log file:

[0x00007ff584ac3c30] session_start() /var/www/web-root/app/code/core/Mage/Core/Model/Session/Abstract/Varien.php:134
[0x00007ff584ac3ae8] start() /var/www/web-root/app/code/core/Mage/Core/Model/Session/Abstract/Varien.php:223
[0x00007ff584ac3998] init() /var/www/web-root/app/code/core/Mage/Core/Model/Session/Abstract.php:84
[0x00007ff584ac3898] init() /var/www/web-root/app/code/core/Mage/Core/Model/Session.php:42
[0x00007ff584ac3760] __construct() /var/www/web-root/app/code/core/Mage/Core/Model/Config.php:1357
[0x00007ff584ac3640] getModelInstance() /var/www/web-root/app/Mage.php:463
[0x00007ff584ac3508] getModel() /var/www/web-root/app/Mage.php:477
[0x00007ff584ac3400] getSingleton() /var/www/web-root/app/code/core/Mage/Log/Model/Visitor.php:79
[0x00007ff584ac32c8] __construct() /var/www/web-root/app/code/core/Mage/Core/Model/Config.php:1357
[0x00007ff584ac31a8] getModelInstance() /var/www/web-root/app/Mage.php:463
[0x00007ff584ac3070] getModel() /var/www/web-root/app/Mage.php:477
[0x00007ff584ac2e98] getSingleton() /var/www/web-root/app/code/core/Mage/Core/Model/App.php:1336
[0x00007ff584ac2d08] dispatchEvent() /var/www/web-root/app/Mage.php:448
[0x00007ff584ac2ba8] dispatchEvent() /var/www/web-root/app/code/core/Mage/Core/Controller/Varien/Action.php:527
[0x00007ff584ac2a88] preDispatch() /var/www/web-root/app/code/core/Mage/Core/Controller/Front/Action.php:69
[0x00007ff584ac29a8] preDispatch() /var/www/web-root/app/code/core/Mage/Api/Controller/Action.php:40
[0x00007ff584ac27c8] preDispatch() /var/www/web-root/app/code/core/Mage/Core/Controller/Varien/Action.php:407
[0x00007ff584ac25b0] dispatch() /var/www/web-root/var/cache/ew/files/69/b9/Mage/Core/Controller/Varien/Router/Standard.php:273
[0x00007ff584ac2458] match() /var/www/web-root/app/code/core/Mage/Core/Controller/Varien/Front.php:172
[0x00007ff584ac22f8] dispatch() /var/www/web-root/app/code/core/Mage/Core/Model/App.php:365

So it seems to be the point at which a session is opened, I am investigating further.