Today I have experienced this issue. Trying to install Imagick on Windows 8, PHP 5.5 x86. I got this message and it took me several hours to fixed it. Try everything I could, however, I always received this error after every try.
The fix for me is super simple that I cannot even imagine. Because the DLL file was downloaded directly from internet, Windows block it as other application. Just right click on the file and unblock, and restart server again.