I just tested the changes in 13.6.8 and the graphics issues with the right toolbar (see above) have disappeared. 
However, the spacer in the left toolbar still tries to show an icon. And I believe I found the reason: In Customize -> Toolbars -> Options the Image State is set to On. Once I switch it to Default or Off, the issue disappears.
BTW I also found that the length of a spacer's label isn't taken into account when calculating the toolbar width:
Don't know it that was intended...