Is this a ransomware?

I just got this window on my main computer:

Apparently, it's from Windows. I checked, and the process EXE file is the original one (efsui.exe or something like that). However, I don't use EFS (I never heard about it). I never encrypted any of my drives.

Is it possible that my system is infected by a ransomware encrypting my files?

Know someone who can answer? Share a link to this question via email, Twitter, or Facebook.