The page cannot be displayed because an internal server error has occured

Questions
1

Sometimes, when we want to go to the “cart”, we get following message: The page cannot be displayed because an internal server error has occured.

How can we solve this?

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This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/83822-the-page-cannot-be-displayed-because-an-internal-server-error-has-occured