I live in Portugal but work some weeks in Norway for a Norwegian employer — where is my salary taxed?

Those Norwegian work days are taxable in Norway from day one. The treaty's 183-day protection only shields you when the employer is NOT Norwegian and no Norwegian establishment bears the cost — with a Norwegian employer, the protection never starts. Portugal, as your residence state, taxes the same income too and must credit the Norwegian tax. What usually goes wrong: nobody tracks the day split, and both payroll systems do something inconsistent. Keep a travel log; it is the document everything else depends on.