Basically I was laying on a big map. I don't know how big since I set it on random. I established some planets but I was loosing money and fell past 500 B.C. Then I went to the screen where you change money regulation. But when I looked the taxes bar said 54 B.C and was higher than money output.
When you fell past -500 BC spending is suspended. Thus the spending slider is out of function until you have > -500 BC. In the previous round you went into debt because you spend too much. This round your spending rate is 0% and thus your income is > 0. You debt is reduced and once you restart spending at the same rate from before you will go into debt again. Does this explain your problem?
Or another idea. Buying ships and improvements is very expensive. The leases can dump you deeper and deeper in dept even with spending sliders at 0%. Conclusion: Don't buy anything or maybe only in very rare circumstances (e.g. to reach a valuable planet before your neighbor gets there).