Why do invoices have different from Classic panel statuses?

There are new invoices statuses introduced in UX1 such as: unpaid, processing payments, closed. 

All the new invoices are created "unpaid". All the invoices that are being in progress (e.g. bank operations) have "processing payments" status. All the paid invoices have "closed" status. 

Where I can find all the invoices?

All the unpaid invoices ("open" in Classic panel) reseller can see on the "Orders and Invoices" page and can immediately proceed to pay those invoices choosing one or several invoices simultaneously.

All the closed ("closed" in Classic panel) invoices as well as all others can be found in Invoice History. In the Invoice History reseller can see both types of invoices: for services that they have purchased from the provider for their own use and for reselling services to customers, 

What to do when "Paytool_status is invalid" issue appears while paying an invoice?

The issue will be fixed. As a workaround make sure that you have “not activated” direct debit payment method deleted.

Where I can find ‘payments made details’ in UX1?

Payments are now attached to a specific document (due to financial compliance). Once an invoice is paid, payments are attached to it.