IGMP is used for multicasting, so if you're not sure what it is, you probably don't need it. I don't know of any particular problem that enabling IGMP routing would cause. However, I have had problems with RRAS when I made configuration modifications that shouldn't have affected anything. I would start with the same first step in any troubleshooting process: reboot the computer. In my experience, no incarnation of RRAS is terribly reliable, and rebooting solves these transient problems.
If that bit of brilliant advice (forgive the sarcasm) doesn't solve your problem, then you can either spend a few hours understanding in-depth how RRAS works so you can isolate the problem, or you can spend a few minutes uninstalling and re-installing it. Good luck! I wish it were something that were simpler to troubleshoot so I could offer more tangible advice.
This was first published in September 2002