NAT stops working, when PIA VPN is connected

[RockNLol]

hi,
I'm new to opnsense and the forum so hello y'all 

After experimenting and learning a lot with DD-WRT on my router I now try to switch everything to a dedicated server with opnsense and I am a bit overwhelmed by all the settings.

At the moment I am trying to NAT a few clients (IoT Devices) over a OpenVPN-connection to PIA. The OpenVPN-connection itself is running fine, and also when I add a new manual NAT rule to NAT certain clients filtered by IP said clients use the VPN connection to connect to the Internet. Unfortunately all other devices told to NAT over the WAN connection can't connect to the Internet anymore. The moment I disconnect the OpenVPN connection NAT over WAN works again (without changing the NAT rules).

Does anybody have a clue with this is the case?