I know this is an old post but I thought It would be best to write a reply here.
I’m using Illustrator CS6 and I’m having the same problem. I cannot change the stroke width to less than 1 pt. Except the problem occurs regardless of whether I type the value in or select it from the stroke weight panel. Also it only occurs when I select two or more strokes of varying width first and try to use the stroke panel to adjust them to the same value.
I tried going to Control Panel > Region and Language and then clicking the Formats tag. Selecting Additional settings and then changing Digit Grouping symbol to a period instead of a comma.
I also tried changing the stroke width to none to try and reset it. Nothing seems to work. The solution would be to adjust each stroke individually but there has got to be a better way
I’ve uploaded a 1 minute youtube video demonstrating the problem. You might have to maximize the youtube video screen to see it more clearly.
https://www.youtube.com/watch?v=cjwQe-qMFRU
Thank you