Upgrades can be tricky. While Adobe has probably tested the upgrade process literally a million times, it’s impossible to account for every different configuration and setup that exist in the wild. So, occasionally, there is a hiccup…a glitch in the Matrix…and a rouge bit gets flipped on a disk and the result is a corrupted file. If that file happens to be your Lightroom preferences file, you may get an error when you try to launch Lightroom. Fortunately in Lightroom 6, Adobe makes it easy to reset your preferences.
In Lightroom 6, there are two preference files. Application preferences that control some startup options for Lightroom, and the user preferences, which represent the customizations you make to the Lightroom views, last used catalogs, and plug-in settings. This reset feature will only affect the user preferences. Catalog-specific settings are also not affected by the reset.
In previous versions of Lightroom, you needed to find and delete your existing preferences file in order to reset and regenerate a new file. That can get messy and complicated since the files are never in easy to manage locations. In Lightroom 6, you simply need to hold down Option-Shift (Alt-Shift on Windows) when you launch Lightroom, and it will prompt you to reset your preferences.
Click the Reset Preferences button to reset your user preferences, or click Start Normally to use the existing preference file.
On the Mac, you may need to restart your system in order for the reset preferences to actually take effect because of the way that the operating system caches the preferences.