I'm afraid gmailfeed will have to be removed from the repos.
Once you enlarge the plasmoid, the red warning triangle is replaced by the actual warning, configure account
. Thus it looks gmailfeed is too old to be compatible with current kaccount. It is build from the very last commit upstream committed ( late 2015), rebuild does not change any.
You can try to report the issue upstream, see if they are willing to make it compatible again.
https://github.com/anthon38/gmailfeed/issues