I'm confused right now. After activating hummingbird the dashboard & hummingbird gzip compression page will be downloaded as an PHP file. So something seems to be odd with the php interpreter at this time ... but because it effects just those 2 pages maybe something within the plugin "hummingbird" will create this issue?
After disabling the plugin everything is working fine again.
I reinstalled wordpress, the plugin and recreated the whole domain via Plesk, so it should not an bug within the database or something like that.
(Support access is activated)