ow to solve the "Error saving model: Index was outside the bounds of the model"

Current version (but the error was also in previous versions).

This error appears whenever I press the "save current model".

When I choose "Save as" it save the model, but the connecting lines are diffused and out of structure.

How to solve this! Can this be solved?

I have no clue where the problem is.

Each model validates as without any errors.