Networking – PPTP connection: Timeout sending config requests when in eduroam network

trying to connect to a pptp-vpn from within my university network ('eduroam' standard). connection cannot be established (LCP: Timeout sending Config-Requests). The connection works flawlessly from any other network. I cannot check the logs on the server's side.

I can connect to port 1723 via telnet.

I provide logs both for successful connection attempts (at home, DSL) and unsuccessful ones (at university); additionally some information about my connection at university (netctl connect profile, ifconfig output, routes) in a gist: https://gist.github.com/Foucl/6b842f94ef5042ba2062

I followed the pptp client diagnosis suggestions and the fault tree there (I am out of links) and have added the iptables mentioned. I could not get hping2 to work (it's not producing any output).