If the configured managesieve host doesn't exist, there is no error reported to the user. When going to Settings -> Manage Sieve Scripts, the progress circle will just go on and on without reporting any errors. A simple typo in the host name is very hard to pin point because of this!
Git commit f3750c52aff3bd7f4c26e272488d60c4450c8230 by Laurent Montel. Committed on 26/10/2018 at 17:46. Pushed by mlaurent into branch 'master'. Fix Bug 391806 - Sieve doesn't error on bad managesieve host FIXED-IN: 5.10.0 M +2 -1 src/kmanagesieve/session.cpp https://commits.kde.org/libksieve/f3750c52aff3bd7f4c26e272488d60c4450c8230