"Something is wrong" with the "Max. Domain" setting, I believe. I often receive errors like the following when changing any information about a site (like traffic limit or something else).
This website already has 13 domain(s). You cannot decrease the number of domains.
Yes. i get this quite often. It appears to make any changes you have made without any problems, and then gives you the error.
The number, in this case 13, is always equal to the number of users and nothing to do with domains.
I haven't used any extra email aliases so can't comment, but will have a play later.