The complete procedure is as follows (this is on Linux, on Windows it should work as well if you change the paths):

Exit Eclipse\\
Delete the directory ~/.eclipse/org.eclipse.equinox.security\\
Create a text file containing your master password, e.g.\\ 
  echo "secret" > ~/.eclipse/master
Add to the very top of eclipse.ini, found in the Eclipse program directory (as two lines, don't combine into one)
<code - /usr/lib64/eclipse/eclipse.ini>  
  -eclipse.password
  /home/user/.eclipse/master
</code>
Start Eclipse again.