When i receive a mail with calendar invitation doesn't matter what action i performed (yes, no) deleting this mail opens a progressbar window that never ends. After that any other mails cannot be deleted till restart KMail and i you try to delete calendar mail makes a loop.
I try to investigate, delete item ends in session delete command that is serialized of generated protocol. So depend of protocol it tries to mark as read to the corresponding server but this cannot be done if your account is not same in current mailbox and in Google. It became endless loop, so *i think* (i'm not seeing in the code) should be try with current account on corresponding server not in google one.
In facts that's account does not include in Kontact, after i did it, does not have such issue, so it's related to accounts that no present in kontact.