For a more graphical program you could try out IONCROSS’ single player (save game editor) that you’ll find -> here
If you are using a mod, make sure to generate the right GAMEDATA files with the given programm (also found on the above page)

Solved the problem.

It seems there wasn’t a problem in game itself, but rather with windows OS. I solved the problem by picking a tip from Microsoft community, to run sfc.exe, to cehck system files for problems. System file checker found some problems and fixed them and now FL works fine.