The following error appears in Lightroom:
"Restart required. Lightroom has encountered a problem with its menus. Some menu commands will not work until Lightroom has been restarted."
This error can occur when you've been using Lightroom for a while without restarting, especially if you've been using many menus. One way that you can use many menus is if you have a large number of Develop presets and you have been right-clicking your photos to perform menu items.
Restarting resets the number of menus Lightroom tracks, allowing Lightroom to function correctly.
After you restart, your Develop presets won't be listed in your context menu. Choose the Load presets menu item to restore your presets into your context menu. Note, however, that this issue might occur again if you load your presets this way.