Edit: Not left side, right side of start menu
Try restarting dwm.exe and/or updating video drivers.
Same problem here, since the new (preview) Windows update, StartIsBack is on the latest version, same as the nvidia driver. Restartring dwm.exe doesn't change anything. If I hold ESC to prevent SIB from loading, it doesn't change anything. We need a hotfix I guess. I also have Fences installed and this guy seems to have the same problem as us here: https://forums.wincustomize.com/501833/issues-with-fences-starting ? I will investigate a bit now.
Ignore the above message, I had a different problem and it was fences.