How to solve this error?

Receiving the following error - 
Fatal error : Call to a member function getId() on null in  /html/shopware/engine/Shopware/Components/DependencyInjection/Bridge/Session.php  on line  59
503 Service Unavailable 

From some of the other forums I understand that this is a caching thing and a setting that needs to be changed. Can anyone tell me how to change this without access to my backend… this error only appears when i try to access the backend.

Many thanks, 

Daniel