The problem is that the algorithm (lines 970-983 in simworld.cc) for generating a list of population sizes is taking an inconsistent set of parameters. In particular, it is currently impossible to generate cities with a population smaller than half of median_population, which you set to 7500. However, in pak128.sweden we have max_small_city_size=3000. This means that it gets stuck in an infinite loop once it starts trying to generate 'small' cities, since each number it tries is at least 3750.
To avoid this, you can set median_population to less than 6000. There should probably be some checks in the code to prevent this situation arising; in the long term I think the city generation algorithm is likely to be rewritten without this code for generating population sizes.