Problem:
Submitting an answer to a numerical question returns a message saying the answer contained improper characters.
Action:
You will get this message if you leave a question blank or you type anything besides numerals and a decimal point in a numeric answer field. Make sure you answer all questions in each block of questions, and don't put any commas or % signs in numerical answers.
You should also avoid symbol characters or other key combinations that don't show a standard character in essay answers.
See also: SimUText will not accept numerals with decimal comma for numeric questions