Besides Beta 3 I tested the upgrade on a copy of my main site and I always had a white page so far after upgrading when I opened my site.
This is what I experienced and how I get it up and running.
I didn't overwrite the files on the server. I deleted all the files and uploaded the new ones.
Be careful: Don't delete your uploads/downloads folder etc.. On my dev site I don't care!
I made sure the background task which is shown in the ACP was executed. I have a very small site and and I just clicked run it now. No, problems there.
I cleared all the caches in the support section of the ACP.
My main app is pages. After the upgrade I had to open every page and remove the used templates. Although, before the upgrade I switched to default templates this didn't help.
I had to go in the the database settings and remove the templates as well.
After these changes my dev site worked.
Of course, none of the custom blocks in pages worked. But this I expected.