Try using the File Open/Save dialog of any other KDE application (Kate, Gwenview, Cantor, etc.) to add/remove entries to Places. They do appear in Dolphin (still Qt4 based) but trying to delete them is ineffective.
2 more notes:
1. This bug occurs with either Plasma 5.2.2 or 5.3.0 installed.
2. Either KF4/KF5-based applications' file dialogs can manipulate Places succesfully.
By the way, this may be a Kubuntu-specific bug, as there's nothing relevant on bugs.kde.org.
Workaround:
Try using the File Open/Save dialog of any other KDE application (Kate, Gwenview, Cantor, etc.) to add/remove entries to Places. They do appear in Dolphin (still Qt4 based) but trying to delete them is ineffective.
2 more notes:
1. This bug occurs with either Plasma 5.2.2 or 5.3.0 installed.
2. Either KF4/KF5-based applications' file dialogs can manipulate Places succesfully.
By the way, this may be a Kubuntu-specific bug, as there's nothing relevant on bugs.kde.org.