On Chrome, you click in the tool thing in the top right of the screen, pass the mouse over the "tools" option and click on the button "Clear Browsing Data". Or just do Ctrl + Shift + Delete.
I forgot that yesterday was the newest release =) and I just opened my newsletter and cleared my cache. I signed in with no problems but the sign in thing should be fixed soon as the images thumbnails should be fixed in the next release.