A gentleman at trainfever replied to me.
Sorry for coming back to you so late. The freeze at the end of the month occurs, when the game has to do various calculations, which tend to get more demanding over time as cities grow. We put a lot of effort into optimizing this calculations.
As for the vehicle window this would need further investigation. If you want, you could send us your save game so we can have a look at it! You can find our email address in the “About / Urban Games” section.
Besides this gem he also suggested that some mods might be causing the problem. So I removed the mods, and uninstalled the game and started with a fresh game. The failures were reduced and the game did run better with no mods. The other action I took was to pause the game when I was adding new lines or vechicles.