Version: (using Devel) Installed from: Compiled sources This is a usability thing. Every once in a while this dialog comes up and demands my attention. I as a user have no idea what that means... Please make it not appear ever again. Link to screenshot: http://dethware.org/images/terrible.png
As a workaround you can disable the option "Include this folder in the mail check" option for that folder. So it won't be checked every X minutes and you won't get that error.
Thank you for taking the time to file a bug report. KMail2 was released in 2011, and the entire code base went through significant changes. We are currently in the process of porting to Qt5 and KF5. It is unlikely that these bugs are still valid in KMail2. We welcome you to try out KMail 2 with the KDE 4.14 release and give your feedback.