The cause of this problem is the account type used for the invoice. If it is a customer invoice, the account type used to save the total value of the invoice (including tax) should be "Receivable" type. For the vendor bill, it should be Payable.  

  the problem  was, that the type of account of  each invoice line shell be of type Income and not of type Receivable !!!!! So, problem solved :)

The invoices get auto paid state if the invoice values will be zero. Odoo9 work intelligently and set Paid state if the invoice value is zero.  I am facing the same issue, but when i check it throughly it gives me this reason. May be you all having the issue due to the zero valued invoice.

I had a same problem. I created new customer and it started to work with that customer. aka (after validation it goes Open state instead of jumping to Paid)   I had to fix Account Receivable account of original customer  - I had a wrong type of account there and I changed it. Now  also orginal customer and invoicing  works.  Accounts Receivable account can be set wrong? If you can test this and verify if this is a reason.