Service Unavailable viewing a customer/order

[Dannywiz]

Hello.
I am experiencing this issue just with some customers. Their records appears normally in the customer list, but as I click on "edit" or "view" I get this:

Service Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

The same result will happen if I try to view an order of that customer.

I have even checked the database, comparing all data form the customer (and his orders) and other customers, in several tables, and nothing appears to be different. 

The customer is able to login as usual, but he gets the same error when trying to see the order details. No error appears ihe clicks on "reorder" instead.

Can someone give me any idea about some check to do again?

Thanks