installation can't connect with WordPress.com anymore

My wp-installation kellerabteil.org can’t connect to wordpress.org or wordpress.com since my server provider moved everything to a new server.

While that looked like the server configuration was making problems it’s probably something else since (my) other wp-installations on the same new server do work properly.

that’s what I constantly get:
trigger_error( __( ‘An unexpected error occurred. Something may be wrong with WordPress.org or this server’s configuration. If you continue to have problems, please try the support forums.’ ) . ‘ ‘ . __( ‘(WordPress could not establish a secure connection to WordPress.org. Please contact your server administrator.)’ ), headers_sent() || WP_DEBUG ? E_USER_WARNING : E_USER_NOTICE );

Same time I had problems with akismet and jetpack. Both were showing same time that my installation should be connected to my wordpress.com account and that the connection is not working. Neither could I disconnect from the backend (nothing happening) nor reconnect.

Deactivating all plugins didn’t help. Activating a built-in theme didn’t work. Doing a manual update didn’t work. Still have the same problem although jetpack finally aknowledges that there is no connection and shows the ‘connect with wordpress.com’-screen. But then there’s a connection timeout and: register_http_request_failed.

Don’t know what else to try. :/