Highlighted content web part error for read only users

I mean, what is the highlighted content settings? If you are scanning a different site that the users do not have access too etc.

Then this would be a correct message. How long ago did you add the users to the content if they infact have access? Takes some time for indexing to happen etc.

Anyway, something tells me there is a permission mishap here.

That's totally unexpected. I would recommen you to open a support ticket and report the problem