D-30 vs D-65 is more than just the $12K gross income threshold. 

Check the instructions for both forms.

It involves whether capital is a materially income producing factor, whether the work is performed by the owner(s), and whether the business cannot customarily be incorporated.

And yes this year DC started rejected efiled extensions if the business is not registered in DC. Sounds like you found out they are rejecting actual returns, too. I haven't got that far yet.