Wingpanel crashes on button clicks

[Paul]

I have recently come across a problem in which Wingpanel crashes whenever a button (such as "shut down", "power settings", an arrow in the calendar) is clicked.

The output when I run Wingpanel in a terminal after killing cerbere (I don't at this moment know how to provide more detailed relevant information) and click "Shutdown" is:
paul@Quellteich:~$ wingpanel
[INFO 19:54:34.239962] Application.vala:153: System Panel version: 2.0.3
[INFO 19:54:34.240045] Application.vala:155: Kernel version: 4.10.0-32-generic
[FATAL 19:54:34.364129] [AccountsService] act_user_get_uid: assertion 'ACT_IS_USER (user)' failed
[FATAL 19:54:34.364517] [AccountsService] act_user_get_uid: assertion 'ACT_IS_USER (user)' failed
[FATAL 19:54:34.366261] string_to_string: assertion 'self != NULL' failed
[FATAL 19:54:34.376949] display_widget_set_icon_name: assertion 'self != NULL' failed
[FATAL 19:54:34.377306] display_widget_set_icon_name: assertion 'self != NULL' failed
[17:54:35.171004 Gtk-Critical] gtk_box_pack: assertion '_gtk_widget_get_parent (child) == NULL' failed
[17:54:35.217209 Critical] string_to_string: assertion 'self != NULL' failed
[17:54:35.383177 Warning] [DesktopFileInfo] Unable to create AppInfo
[17:54:35.396333 Warning] [DesktopFileInfo] Unable to get exec for Other
[17:54:41.082798 GLib-GObject-Warning] cannot register existing type 'GdkWindow'
[17:54:41.082927 Gdk-Warning] /build/gtk+2.0-jJZIsQ/gtk+2.0-2.24.30/gdk/x11/gdkdrawable-x11.c:952 drawable is not a pixmap or window
[17:54:41.083002 GLib-GObject-Warning] cannot register existing type 'GdkWindow'
[17:54:41.083038 Gdk-Warning] /build/gtk+2.0-jJZIsQ/gtk+2.0-2.24.30/gdk/x11/gdkdrawable-x11.c:952 drawable is not a pixmap or window
[ERROR 19:54:41.085132] [Gdk] The program 'wingpanel' received an X Window System error.This probably reflects a bug in the program.The error was 'BadWindow (invalid Window parameter)'. (Details: serial 4057 error_code 3 request_code 20 (core protocol) minor_code 0) (Note to programmers: normally, X errors are reported asynchronously; that is, you will receive the error a while after causing it. To debug your program, run it with the GDK_SYNCHRONIZE environment variable to change this behavior. You can then get a meaningful backtrace from your debugger if you break on the gdk_x_error() function.)
Trace/breakpoint trap (core dumped)

I believe this may be related to the icon-hovering crash in Slingshot - I got them around the same time. The output of such a crash is:
paul@Quellteich:~$ wingpanel
[INFO 19:57:46.286729] Application.vala:153: System Panel version: 2.0.3
[INFO 19:57:46.286842] Application.vala:155: Kernel version: 4.10.0-32-generic
[FATAL 19:57:46.398751] [AccountsService] act_user_get_uid: assertion 'ACT_IS_USER (user)' failed
[FATAL 19:57:46.399302] [AccountsService] act_user_get_uid: assertion 'ACT_IS_USER (user)' failed
[FATAL 19:57:46.401011] string_to_string: assertion 'self != NULL' failed
[FATAL 19:57:46.433257] display_widget_set_icon_name: assertion 'self != NULL' failed
[FATAL 19:57:46.434027] display_widget_set_icon_name: assertion 'self != NULL' failed
[17:57:47.219511 Gtk-Critical] gtk_box_pack: assertion '_gtk_widget_get_parent (child) == NULL' failed
[17:57:47.422338 Warning] [DesktopFileInfo] Unable to create AppInfo
[17:57:47.443121 Warning] [DesktopFileInfo] Unable to get exec for Other
[17:57:50.542346 GLib-GObject-Warning] cannot register existing type 'GdkWindow'
[17:57:50.542467 Gdk-Warning] /build/gtk+2.0-jJZIsQ/gtk+2.0-2.24.30/gdk/x11/gdkdrawable-x11.c:952 drawable is not a pixmap or window
[17:57:50.542688 GLib-GObject-Warning] cannot register existing type 'GdkWindow'
[17:57:50.542750 Gdk-Warning] /build/gtk+2.0-jJZIsQ/gtk+2.0-2.24.30/gdk/x11/gdkdrawable-x11.c:952 drawable is not a pixmap or window
[ERROR 19:57:50.547124] [Gdk] The program 'wingpanel' received an X Window System error.This probably reflects a bug in the program.The error was 'BadWindow (invalid Window parameter)'. (Details: serial 3948 error_code 3 request_code 20 (core protocol) minor_code 0) (Note to programmers: normally, X errors are reported asynchronously; that is, you will receive the error a while after causing it. To debug your program, run it with the GDK_SYNCHRONIZE environment variable to change this behavior. You can then get a meaningful backtrace from your debugger if you break on the gdk_x_error() function.)
Trace/breakpoint trap (core dumped)

I have recorded a demonstration of both problems: https://youtu.be/UUOInNFJQBI
I would also include a list of relevant .desktop files, if I could find which directories are checked.
A list of installed packages is attached.
Elementary Tweaks was installed, but removed.