What is your version? What is the speed you are trying to change it to? v19.04.30 and later contains the fix mentioned here. However, even in these versions, we found a bug that setting the speed too low is causing a crash. This low speed value was easily achieved by using the arrow buttons on the field or the mouse wheel. That was fixed in the v19.07 beta.
It can also be that some clips for some reason cause a problem. Do you always try with the same clip or the same kind of clip (e.g. same camera or capture tool)?