The most common cause for this is your Windows region/language setting. This setting determines whether you use commas (,) or full stop (.) as your decimal separator. Some countries tend to write 1.01 and others 1,01.
So if you enter 10.01 as a dimension where your language setting is a comma, DSM would refuse the change as Windows enforces it system-wide.
If you prefer the full stop to specify decimals, then you can change this in Windows region settings:
Go to Control Panel » Region and Language » Under the Format tab select Additional Settings » Under the Numbers tab change decimal symbol from a comma(,) to a full stop(.).