Elias froze. He had bought the software legally years ago, a one-time purchase of a perpetual license. But Adobe had long since retired the activation servers for the Creative Suite era. The software was phoning home to a server that didn't exist anymore to verify a number it no longer recognized.
If you previously purchased a perpetual license for Acrobat XI Pro, you can still recover your 24-digit serial number through these official channels: adobe acrobat xi pro 1100serial number top