I use Avaya One-X for work. I got the above error this morning. I couldn’t find a resolution via Googling, and Avaya’s support is notoriously sucky. BUT, I did find out what the purpose of Preferences.xml is. It keeps settings like window positions. Bearing that in mind, I figured before reinstalling I’d try blowing it away (renaming it, actually) to see if it would recreate. It did, and the app came up fine.
Preferences.xml is located in
%APPDATA%\Avaya\one-X Agent\2.5\Profiles\[Profile Name]