Myron,
Got a few questions that should help us track this down:
1) Did you upgrade your existing Oxygen install to Photon or did you create a new Photon install?
2) Do you see this error on every start?
3) This functionality is provided by the CSS Intellisense code extension – by any chance did you make other changes to your workspace or environment? Bring in additional projects, CSS files, etc.
4) If you have the time, could you try to open a new workspace and create some dummy project with a few CSS files in it, do you see the error there too?
5) Do you have VSCode installed, have you used it recently?
Looking for external changes that could have caused this – the Photon update doesn’t seem like a likely culprit at this time.