I just reinstalled Mumble, and now when I try to login on my favourite server I get «Incorrect Password». Why is that?
When you install Mumble for the first time, a certificated was created for you. That is your "password." You should've saved it somewhere safe, and reimported it when reinstalling Mumble. You didn't.
When you registered, or were registered with your favourite server, what actually happened was a binding between your certificate and your nickname. With that certificate, you can only login with that username; without that certificate, you cannot login with that username. You now need that certificate.
If you still have it saved somewhere, import it: open Configuration → Certificate wizard, then pick Import certificate. Select your certificate and continue; you'll be able to login normally.
If you don't have the certificate anymore, because you probably never even knew about this whole thingadongdong, what you must do is:
...and next time, remember to import that certificate! It's important.