vpn thru linux firewall

I have setup a linux (RedHat 7.1) firewall/router for my dsl connection. I
can open an IPSec vpn to my office from a MS Windows Xp system and every
thing works okay. If I open an IPSec vpn from another linux (RedHat9 or
Mandrake 9.2) system, it appears to open okay but I can no longer
communicate.

I have no problem communicating as long I do not connect the IPSec vpn,

I have set the linux firewall/router to be wide open for testing and this
does not help.

If I do the same thing through my old Hawking PN9235 router everything works
OK for both Windows and linux.

Any suggestions as to what might be wrong.