Registration at the commune

Hi all, I'm in a bit tricky situation with the commune registration and I'm sure someone must have been in similar position before me.
I have arrived to Brussels for a new job last saturday and I'm staying at airbnb for now. I signed the employment contract on Monday and was told by the HR that I have to register at the commune in order to be on the payroll,  pay tax and receive my salary. I also read on the Forest commune's website that I must do this within 8 days, and without much hesitation I went and registered. Today I was told by the house owner that the police was checking for me at the address and that it's not possible to register at temporary accommodation.
I find this very strange that the local authority asks for 8 days limit while at the same time requiring permanent residence - nobody finds a flat in their first week, right? 
So I'm wondering what happens now? I have my national insurance number and a piece of paper that I declared my residence, but the police verification has failed. I assume that I will have to register at another commune once I find a permanent flat. But will I be able to do this while there is an open application in Forest? Or will the commune in here simply delete my file? What happens in a situation like this?

I'd be grateful for any tips.