I do not believe the cause is Connections. The primary reason is that Connections Categories are not the same as WordPress Categories. They are in fact saved in entirely different places in the WordPress database … Assuming ‘Intern nieuws’ is a standard WordPress category and you are using the WordPress Category Menu item to add the category to the menu.
Could you confirm the act of adding an entry does indeed cause this change? Fix the category menu and do nothing else but add a new entry. If it does, then I’ll need a WP admin login and a FTP login to investigate further because this should literally be impossible since the data is saved in two different locations so having the same ID will not matter, In addition they share no code at all.