I recently configured my ASA to use my internal DHCP server to hand out addresses instead of the local DHCP pool that I had originally created. The issue is that when I'm using the DHCP server to hand out the address I can't access any internal resources. If I use the pre-configured VPN DHCP pool on the ASA, everything works fine. Note that the same NAT rules apply (I'm using the same address space)
I noted you trying to do a DHCP proxy RFC 3011 and RFC 3527. please have sure your ASA can reach the DHCP server. also i belive you have define your DHCP external server ip address in your tunnel group policy. cisco already release a document with this problem.