It seems uBo blocks some sort of log thingy that prezi.com uses every 10 seconds or so. When it is blocked, Prezi thinks you are not connected to the internet and it keeps you from using things that require an internet connection, such as uploading your own background image.
There are no complicated reproduction steps. Simply go into edit mode on any Prezi Next, and notice the cloud status icon on the top left of the page is stuck on "reconnecting". It tries it every couple of seconds, you can see this in the number next to the uBlock icon and the log entries incrementing every few seconds. The connection status returns to normal the moment you disable the uBlock for the site. I haven't tried it on a normal Prezi but I imagine it's the same thing.
yep, I tested it, that filter seems wrong. An easylist author has been notified (and I sent him your credentials, so ... wait until we get a positive response)
I'll fix the issue temporarily in uBo (unbreak list)
