Debug information from libcalendaring ends up in the IMAP communication.
Possible solutions are:
* patch libcalendaring to not have any debugoutput
* recompile libcalendaring with QT_NO_DEBUG_OUTPUT QT_NO_WARNING_OUTPUT
* patch libkolab or the calling application to install a custom QtMsgHandler
* catch this in wallace and Roundcube
Because only a messagehandler makes sure that no debug output makes it to stderr and is thus captured by when piping something through a command line utitlity
such as kolab-format, the best solution is to install a custom message handler in libkolab (nothing uses libcalendaring without libkolab).
Potentially relevant issues:
https://issues.kolab.org/show_bug.cgi?id=4934
https://issues.kolab.org/show_bug.cgi?id=4580
https://issues.kolab.org/show_bug.cgi?id=3742
https://issues.kolab.org/show_bug.cgi?id=5370