An error occurred while Internet Connection Sharing was being enabled (null)

You have access to a LAN port with access to the internet, and you want to share the internet connection wirelessly (ad hoc network).

So you go to Network and Sharing Center, do the usual to set up the ad hoc wireless network, and accept the offer to share the internet connection (wow, Windows 7 knows I want to share my internet connection). Unfortunately, a message pops up saying that it cannot be enabled at this time.

Being somewhat familiar with Windows, you launch services.msc, and check that the Internet Connection Sharing service and all its dependencies are enabled. You try starting the service, and it stops automatically with something like:

The service started and then stopped. Some services stop automatically when they are not in use.

Fair enough. You retry creating the ad hoc network, and it fails again at the last step. You test the ad hoc network with another PC; it is working, no doubt, just that there is no internet connection.

Now you launch ncpa.cpl, and try to share the internet connection from the LAN directly. You get the error:

An error occurred while Internet Connection Sharing was being enabled (null).

At this point you give up. You want to call IT, but are pretty sure they do not advocate sharing the internet connection. Then you have a stroke of inspiration, and reopen services.msc. Scroll to ICS, and realize that the startup mode is Manual. You change it to Automatic, and retry.

Oh! It worked!

PS: your login account needs to have admin rights. That is beyond the scope of this article.