Bump.. I got the same issue.. Started getting unknown class errors out of nowhere.
Eventhough I have all classes loaded properly via composer or custom autoloader I get these :
"Use of unknown class: 'Some\Class 'PHP(PHP0413)
errors.
Like Filip says the error goes away if I rollback to previous version.
I think I've tracked it down to 1.38

If I open the said class file in the workspace, the error goes away.
But thats not ideal having all files open in the workspace so I am rolling back to the last 1.37.xx version for now. I dont know whats changed but can you look into it


I dont think its my namespace mapping, because it happens with common Wordpress classes too.