Thanks for the report, I think it is fixed now.

I just implemented that feature, let me know if it causes problems.

It could be an issue the birthdates with the littermates, they have to have the same birthdate or no birthdate to show up.

If you need help, please give more information on the conditions when the error occurs, like exactly what information about the dog you entered before saving.
I need to know more to find what the problem could be.

No, it should work. Every group has to be a child of another group except the root.
Just rename the root to Kennel or whatever you want and add a child group and so on.
Look at our kennel to see how you can arrange the sub-groups under the kennel node:

http://www.dogtec.com/kennel/noatak/kennel

This is what we use too without any problem.

No you cannot, all you can do is create a dog for each littermate in your own kennel and link its parents to dogs in other kennels.

You just have to type the first letters of the name in the text box to the right of the select when you can choose a father or mother from your kennel. After a second or so, a list will pop up with matching dogs and their kennel and you can choose from that.

There is already an active field for the dogs so that an inactive dog will be hidden.

Yes, there are quite a few of those around.