I meet the same problem in Window 8.1 64bit , Ruby 2.3.3 64bit , Rails 4.2.10.
My quick solution :
Step 1 : Remove all bcrypt in gem : gem uninstall bcrypt
( choose option 3 if existed (remove all))
Step 2 : Install bcrypt again : gem install bcrypt
Then check version of bcrypt : gem list bcrypt
Step 3 : In Gemfile of your project, add : gem 'bcrypt', '3.1.11', platforms: [:ruby, :x64_mingw]
notice : change the suitable version of your bcrypt (my version : 3.1.11)
Step 4 : Run command in the path of your project : bundle install
Step 5 : Remove the unneeded bcrypt :
Run command : gem uninstall bcrypt
You can see some version of bcrypt , you should keep one version : bcrypt-[your-version] . (Remove all version : bcrypt-[your-version]-x64-mingw32 . In mycase , remove : bcrypt-3.1.11-x64-mingw32)
Restart Rails sever , done
I think this proplem because when we install bcrypt , we didn't specify 64bit or 32bit , it is not compatible with window or ruby ( 64bit or 32bit )