PPTP utilizes the GRE (Generic Routing Encapsulation) protocol for its point-to-point tunnel. As a pure IP protocol GRE uses only IP addresses but no port numbers giving the router’s NAT a tough time to track such a connection. In its base configuration OpenWrt Backfire is able to NAT a single PPTP connections but not multiple such connections concurrently. It is also unreliable when trying to establish consecutive single PPTP connections from different LAN clients in rapid succession. This limitation can be lifted (as far as I could make out so far) by installing the following package.
For the current versions of OpenWRT (since Chaos Calmer 15.05), you should install:
opkg install kmod-nf-nathelper-extra
You should now be able to use multiple PPTP connections from LAN to WAN at the same time.
Old versions until Barrier Breaker 14.07 used ‘kmod-ipt-nathelper-extra’ instead:
opkg install kmod-ipt-nathelper-extra