I am going from memory but I think last year, on one occasion, I had the same problem. I THINK I was successful when I entered 2 zeros first followed by my 4 digits.
Be careful about differentiating between State Pension (which is included in the taxable income) and Government pension ie Civil Service which is excluded.
I agree that Bangkok is a bit of a Marmite thing.....................it took me several years to like Bangkok - and that came once I learned more about the variety that Bangkok offers.