I recently upgraded a site from V2.4 to V2.6 and everything works fine until a ucstomer gets to the confirmation stage of a purchase when there is an Internal Error message. When I go into admin and try to configure a payment method I also get an Internal Error message. I get this error whatever payment method I try to look at.
Execution of the child request failed. Please examine the InnerException for more information.
This occurs when moving from the shipping to the payment stage.
There are also 3 repeated warnings which might be relevant.
Resource string (shop) is not found. Language ID = 1 Resource string (membership) is not found. Language ID = 1 Resource string (links) is not found. Language ID = 1
Only other thing I can think of is to look in System -> Warnings and see if there are any clues. (The Warnings feature is new in v2.6 I think - it performs some basic checks on files, permissions and config settings and reports any issues that it finds)