Hi, this post relates to 1.7.6.2 and above. Not sure about below.
The error is normally caused by imported data from other carts like ZEN cart and Open Cart. They do not have the fields that Presta shop requires. Hence when you click on a imported customer then the details are not found and a MySql error occurs - hence ERROR 500.
Most of this is imported old orders which have prestashop mysql fields in ps_orders set to zero.
Try this but BACKUP YOUR DATABASE FIRST.
From the Advanced Paramaters tab on the left menu select database
Select SQL Manager
Create the following indiviually and then action with the cloud icon. You may get an error of 500 on each action but do not worry.
Name SQL STATEMENT
roundtype UPDATE ps_orders SET round_type = '1'
rates UPDATE ps_orders SET conversion_rate = '1.000000'
round mode UPDATE ps_orders SET round_mode = '2'
Payment by PayPal UPDATE ps_orders SET payment = 'Payment by PayPal'
Site UPDATE ps_orders SET id_shop_group = '1'
recycle UPDATE ps_orders SET recyclable = '0'
Once these MYSQL actions have happened you should be able to edit your customers.
This worked for me, after a zencart import of orders.
Let me know if it works for you....
Doctoremote