does the remote subnet you are connecting to use a network address that conflicts with the remote subnet access via IPSec ?
Or does the remote VPN server public IP conflict with an address used in the IPSec config ?
If I understood your response correctly there are no conflicts. I have similar config at another site running 3.x version of the FW and I do not see this problem.
Is the remote PPTP VPN server ( endpoint ) the same IP address as an IPSec endpoint ?
If so this will cause issues as you would have two routes to the same destination.