Can't sign with digital ID

I'm using Adobe Acrobat Reader DC version 2019.021.20061 on Windows 10 Enterprise.

I've successfully set up my digital ID and certificate. I click but when I drag my mouse to create a signature box and digitally sign, the whole screen becomes dark and signature won't appear.

I could sign before, but don't know what has happened. My digital ID is still valid, so really don't know what has happened! I'd reallt parreciate if you help me with this.