this seems to a problem with the SASDM, the Enhanced Editor on Windows. I have seen this issue in the past due to several reasons. I could imagine there is something wrong on a library (mainly when you use a non-English language/locale) or the Windows Registry.
I would suggest the following approaches, in order. If one works, you won't need to take the following ones:
1- Check the latest hotfixes available for your installation, and install if any is relevant to your issue.