I am working on a business that is not really able to produce any money yet and I am focusing on the contacts, business model and a lot in the product itself but I will probably start a normal job in the summer to have some income again.
Does it make sense to register the company before it is ready?
What problems/issues could I have when I start working for a company again? I will certainly mention it. Could it be a problem to be employed or even to work in parallel in my company?
Whether or not your new employer will allow you to work o the company will depend on many items - such as the business both companies are in, company's rules etc etc
Therefore, you're able to freelance for any company, no need to be an employee bounded by clauses in the contract.
If you present yourself as freelancer, it's totally transparent that you may work for other people, may have other sources of income, potential conflicts of interest, etc. So, anything not restricted in your freelancer agreement is OK to do.