When you insert a new line in comments with F3 key, navision inserts a new line. If you are with line 10000, navision inserts line 5000, 2500, 1250, 625, 312, 156, ... and continuing to line 1. Therefore, Navision makes an error, because you are between line 2 and 1.
You can go back to the beginning cutting and pasting all comment lines. Navision renumbers lines with 10000, 20000 , ..., and you can continue working normally.