Remote WHMCS host settings not saved

After initial installation we entered our WHMCS host URL and clicked the 'Save Settings' button. After this we got an empty page. When refreshing this configuration page the WHMS host URL is still empty.

When trying to load our sample page wiht shortcode '[wcp_content]' we get the error message 'WHMCS Integration: A valid URL was not provided.'

We are not able to configure the connection between wp and whmcs.

Any hints?