I updated Lightroom 5.5 to 5.6 (Windows 7). Now, unfortunately I cannot start the LR program at all. The message: "unexpected error opening catalog" pops up. And all the subsequent Options do not work. The Option "choose a Different Catalog" is useless, and the "try again" just as much. And "Exit" seems to work, but increases my frustration.
Moving this discussion to the Photoshop General Discussion forum.
I have 5.5 and have the same problem.
I had the same issue - after installing Lightroom 5.6 (64 bit version, Windows 8.1) worked for a couple weeks, then I got the unexpected error message this morning. I was going to install a previous version, like 5.5, but it appears Adobe released 5.7 last week so I downloaded the new one and I'm up and running here now! Wish you the best.