Had the same issue. I was trying to have my daughter icloud acct on my Mail app. When i set this up mail would not let me change the description, says cannot leave the incoming server blank as well. This is grey-out and can't be altered. I could not fix this issue either but I did change the name of my icloud account to icloud. Then deleted my duaghters acct and remade this acct and it assigned her e-mail acct as the decription of the her mail account and not icloud. ie... "Daughter@icloud.com" I noticed that the e-mail server is the same as yours "p05-imap.mail.me.com" so may be an issue on Apple Server and not in the mail app.
Thanks for your reply Paolo, at least I know now that I'm not the only one with that issue.
I couldn't quite follow your description but I'm pretty sure that the fact, that you see the same mail server does not mean much in this context. A crashing mail client cannot be explained from the server side. Even if the server would be the root of the problem, a client should never crash, just because a server is misbehaving.