I have created custom attributes in Mediabay, but then I was trying to do factory reset by deleting mediabay3.db from: /Library/Preferences/Steinberg MediaBay Server
When we edit a preset and add a custom attribute value to it, this value is written in the preset file as well, not just in the mediaBay.db. So, even when you delete the database, upon re-scanning, mediaBay will try to import all the connected info of the preset back. So, I have to guess that it’s during this process that the issue appears. I don’t know what went wrong in your case, I reply just to clarify that deleting the database doesn’t necessarily mean that all attributes will be “reset” as well ( not just user defined, remember that many presets have attributes such as category, character etc).