In this tutorial I will show two easy steps which can solve the prestashop admin 500 server error when you try to update an order status. This error can occur from many reasons, if it is a core problem maybe you should go on the prestashop form and post there your problem, maybe somebody can help you to solve the problem, but before doing that you can try the following steps:

Go to Shop Parameters then Orders Settings then Statuses try to do a Save in that page and check again to update a status for an order to check if the issue is solved. If this mothod is not working, you have to check if the invoice is already generated. 

If your prestashop does not generate the invoice when you will try to change the status for the order a 500 server error will come up. So, if this is the problem try first to generate the invoice for the order and then change the status, it will should work properly. Let me know if any of those methods worked for you in the comments section, also if you encounter any problems you can let the issue here and we will try to help you solve it.