Yes, I have checked that website this afternoon, but I didn't really find what I was looking for.
Actually, after I posted, I've found the answer to my question
It lied in the "Convention Between Canada and the Kingdom of the
Independent Personal Services
1. Income derived by a resident of one of the States in respect of
professional services or other activities of an independent character shall
be taxable only in that State unless he has a fixed base regularly available
to him in the other State for the purpose of performing his activities. If
he has or had such a fixed base, the income may be taxed in the other State
but only so much of it as is attributable to that fixed base.
2. The term "professional services" includes especially independent
scientific, literary, artistic, educational or teaching activities as well
as the independent activities of physicians, lawyers, engineers, architects,
dentists and accountants
So yes, I should pay Dutch taxes over this Canadian income
I'll consult my Dutch accountant (my husband ) to see the correct procedure When I asked him the question this morning, he couldn't really answer (as he didn't know the Convention) but now I'm sure I'll find my way through the maze
Thanks again for your help!