Within the USA, I believe the IRS starts giving you extra scrutiny if you go three years without a profit. I believe in general small businesses income tax returns are scrutinized more closely. So although I'm sure you're being absolutely truthful on your tax returns, this is another reason to be extra truthful.
Is there a reason why you want to roll the previous 401k into a Solo 401k instead of into a Traditional IRA. Are you preparing to do backdoor Roth contributions?
I have run my own side business (I'm the owner and only employee) for 20 years now. I'm a sole proprietor and include schedule C on my federal taxes. My business had several years of negative net income at the beginning. I have to kick in taxes, of course, at the state and city level. All I had to do was get a vendor's license from the Secretary of State to do business as, which I think cost maybe $20. Because I'm a service provider, I have to collect, report, and pay sales tax to my state.