We're experiencing the same issue. A "unexpected error please contact" message from both the API and from the Appstore dashboard. The last successful report we received was on 18th July.

Same issue since 23:00 BST

We were having the same issues since Yesterday. But seems that is working again. I've managed to download the salesReports via the API: https://api.appstoreconnect.apple.com/v1/salesReports