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.
I upgraded form Oxygen to Photon.
I have one workspace that seems to get this every time on startup, my other workspace gets it at least once or twice a day, but I try not to restart a lot.
It is possible I have new CSS files, there are a lot of people contributing to our shared repository and most of it is not under my control.
I do have VS Code and had it open Friday viewing other projects. I saw the error when I started my workspace this morning and that was the first time after a clean reboot (I shut my laptop down on weekends).