[thomb@bossoft.com]

The issue you likely were having is that the button's HTML changes when your mouse is over it.

It goes from having a classname of btnMouseOver to simply btn when the mouse isn't over it. Using the mouse hover trick is going to get the mouse over variation. When you run / try that command, since the mouse isn't over it, that element no longer exists, the command fails.

Your command excludes the part of the findstring that was detecting these changes. Since you filled in email address and password, the button is the next Input element on the page, so that findstring is perfectly valid.