JetPack problems with widgets.php

I have problems with jetpack (2.9.3 and earlier version) since a week by now and I can’t get it clear what it is. It worked OK for a while.
Yes, JetPack is authorized by wordpress.com.

The problem suddenly ocurred in WP 3.8.3 and also in WP 3.9

When clicking appearance -> widgets, … and accessing /wp-admin/widgets.php, I’ll get

Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /wp-admin/widgets.php.

Reason: Error reading from remote server

Does anybody know what to do?