Many people must have already been aware of this problem/solution, but for those who don’t know, read on to know more.
To fix it is rather easy:
1. download the latest version of LESS from the official website: http://lesscss.org/
2. go to C:\Program Files\SimpLESS\Resources\js (or where you have installed SimpLESS, replace less.js with the one you just downloaded
3. now if you try to close and open SimpLESS again and try to compile Bootstrap LESS CSS, it should be working :)
Hope this helps <3