The email can't be a foreign email address ending with Out of curiosity I tried with some other domain I have around ending with different geographical TLDs and the error is always the same. Support mentioned that it’s due to “regulations” but I find this very hard to believe.
In the meantime, you might take a look at Open Tax Solver:
https://opentaxsolver.sourceforge.net/
It is a local program, so it won't care where in the world you are when you are using it.