DSL via VPN/pptp; connection breaks up after a few seconds

[IlikeITcrowd]

Linux Mint 18.2 Cinnamon

Hi,

the problem is, that the connection breaks up after 3-20 seconds (but is working in that time: I can check emails, load webpages ...).

The problem ocurred after trying to install a ProtonVPN https://protonvpn.com/support/linux-vpn-setup/.

First, I set up a new connection (openVPN) and imported the given ProtonVPN-configuration.
I than tried several ProtonVPN-servers, but I couldn't get a connection.

And also, now the main problem with the older VPN/PPTP came up.

With a Mint 17.3 Cinnamon on another partition, the same settings (VPN/PPTP) are working fine.
I installed a Mint 20.1 Cinnamon today, and the settings do not work there either, which really doesnt't help me understand this.