[marcus_holmes]

Those clauses are really hard to enforce generally. They're effectively restraint of trade. Your time not working for your employer is your time. Your employer is on dodgy ground specifying what you can do in it. At least that's the interpretation most places I've worked (UK and Australia). Germany may be different.

But specifically if you were taking an interview rather than working a side job you could make the argument that the clause didn't apply in any case.

It all comes down to how much you pissed off your ex-employer by leaving, and how much they want revenge. No-one is going to gain anything from suing you after you've moved jobs.