If the Identity is configured to place sent emails in a specific folder and this folder points to the Sent Mail folder of the GMail resource the following error occurs: "Sending succeeded, but failed to move the message to the sent-mail folder. Moving items into virtual collection is not allowed". This leaves the email in the outbox which results that another attempt is made to send the email and move it to the configured sent-mail folder. And this goes on and on and on ....
Actually the agent is crashing upon access any emails in the sent-folder of the IMAP account and not just when email is retrieved.
This bug has only been reported for versions older than KDEPIM 4.14 (at most akonadi-1.3). Can anyone tell if this bug still present? If noone confirms this bug for a recent version of akonadi (part of KDE Applications 15.08 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.