Hi,
Sorry for the delay on investigating this problem. I don’t know exactly the root problem (still can’t reproduce it), but looks like it’s related to less and using ** in comments (check this https://github.com/shellscape/postcss-less/issues/138). In that case file name provided by postcss-less is not a real file name but <input css 1>.
So on your side you can try to find such code in your code base (//**).
Anyway there is also bug on our side, we try to get file based on provided filepath (<input css 1> in this case) before checking rule key (parsing error in this case).
We will try to release a patch asap.
Ticket to follow -> https://github.com/SonarSource/sonar-css/issues/208