Could be related to recent changes to the way password strength is calculated. It is done "on the fly" now. Does it make a difference if you disable the password strength column? (That is, make sure that View > Show password strength is not checked.)
Pfewww. I totally frogot to reply to that. So in fact, it's even blinking when the "show password strength" option is deactivated. However after some tests, it's not the only application that behaves this way on my computer. Maybe it's because i have modified a lot of things on my system (graphically speaking) or because…