Fedora 29/Plasma/KDE hangs on logout

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I recently did a system upgrade to F29 (configured for KDE/Plasma), but now when I try to log out, the screen hangs, not returning me to a logon screen. If I Ctrl-Shift-F2 and log into root, I can killall -u and then kill sddm-helper, and I get the logon screen. This doesn't seem to happen on the laptop that I upgraded the week before.

Does anyone have any pointers? This is really annoying! :-) I'm generally technically competent (just not in this particular area), so you can throw technical stuff at me, and I'm willing to help in any way that I can.

If it helps, this is the system log after my most recent logout attempt:

Feb 25 19:33:02 cegis11 ksmserver-logout-greeter[6145]: file:///usr/share/plasma/look-and-feel/org.fedoraproject.fedora.desktop/contents/components/UserDelegate.qml:31: ReferenceError: model is not defined
Feb 25 19:33:02 cegis11 ksmserver-logout-greeter[6145]: file:///usr/share/plasma/look-and-feel/org.fedoraproject.fedora.desktop/contents/components/UserDelegate.qml:31: ReferenceError: model is not defined
Feb 25 19:33:02 cegis11 ksmserver-logout-greeter[6145]: Empty filename passed to function
Feb 25 19:33:02 cegis11 ksmserver-logout-greeter[6145]: file:///usr/share/plasma/look-and-feel/org.fedoraproject.fedora.desktop/contents/components/UserDelegate.qml:31: ReferenceError: model is not defined
Feb 25 19:33:03 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 21585, resource id: 2097267, major code: 18 (ChangeProperty), minor code: 0
Feb 25 19:33:03 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 22147, resource id: 35651591, major code: 18 (ChangeProperty), minor code: 0
Feb 25 19:33:03 cegis11 plasmashell[5444]: QQmlExpression: Attempted to evaluate an expression in an invalid context
Feb 25 19:33:03 cegis11 sddm[1247]: kwalletd5: Checking for pam module
Feb 25 19:33:03 cegis11 sddm[1247]: kwalletd5: Got pam-login param
Feb 25 19:33:03 cegis11 sddm[1247]: kwalletd5: Waiting for hash on 6-
Feb 25 19:33:03 cegis11 sddm[1247]: kwalletd5: waitingForEnvironment on: 3
Feb 25 19:33:03 cegis11 sddm[1247]: kwalletd5: client connected
Feb 25 19:33:03 cegis11 sddm[1247]: kwalletd5: client disconnected
Feb 25 19:33:03 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 22151, resource id: 25165840, major code: 18 (ChangeProperty), minor code: 0
Feb 25 19:33:03 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 22210, resource id: 29360133, major code: 18 (ChangeProperty), minor code: 0
Feb 25 19:33:03 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 22269, resource id: 39845895, major code: 18 (ChangeProperty), minor code: 0
Feb 25 19:33:03 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 22273, resource id: 27262996, major code: 18 (ChangeProperty), minor code: 0
Feb 25 19:33:04 cegis11 kwin_x11[5440]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 22586, resource id: 37748741, major code: 18 (ChangeProperty), minor code ...