Upgrade from 1.7.6 to 1.8.4

[Fredrik N]

Hi

I run the upgrade in 1-click upgrade. It went well.
When I try to use the Front end I get  the error 
 

1/1) ClassNotFoundException

Attempted to load class "Order" from the global namespace.
Did you forget a "use" statement?

in Cart.php line 2251

 

When I use the backoffice, where ever I go  I get the errormessage

Attempted to load class "Order" from the global namespace.
Did you forget a "use" statement?

 

ClassNotFoundException

in var/cache/dev/namespaced_class_stub.php (line 160)

require_once()in classes/PrestaShopAutoload.php (line 160)

 

I tried to clear the cache in BackOffice but I can't do it. I get the above errormessage.
I cleared the cache in webbrowser and  I have deleted the files in var/cache/dev

 

Nothing works
What can I do to get it work again?

 

Thanks
Fredrik

 

How can I correct this error

[Fredrik N]

I found the problem!

I had an override in class Order (override/classes/order/).
I removed it and then backoffice worked.
I had still problem with frontend. I deactivated the module "Cross selling". 
Then it worked!
Then I activated module "Cross selling" and front end still working!