I don't really know what happened, yesterday everything was working perfectly. Today, while working on a new website, I realized that the Media Library is not working. I had the Add New button, but wasn't working, and the page was not showing any image from the library. I've checked on other websites in the network, and the issue was on all of them. I've deactivated all the plugins - and Media Library was back again. I activated the plugins one by one, until I got to Ultimate Branding. Once clicked on "Network activate" I got a blank page, with no access to Dashboard. I clicked back... used the History, no access to the dashboard. Using an FTP client I've changed the name of the folder of Ultimate Branding. Once done, everything is back to normal. Just that now I can't use Ultimate Branding anymore.
Does this happen to any of you? Is there any way I can fix that?
Thank you for all your support!