Acorn ( ), my favorite image editor, was giving me trouble. Whenever I’d open a file in the app, the image would appear immediately—but then the app would hang for five, 20, even 60 seconds. I saw the spinning beach ball of doom, and couldn’t do anything else in the app. I was running the latest version of Lion and the latest version of Acorn, and I checked with a few of my fellow Acorn-using Macworld coworkers; not one of them was having my issue.

Acorn is developed by Flying Meat Software, so I reached out to the company for tech support. After a lot of investigative back and forth, Flying Meat’s president Gus Mueller figured out the solution, discovering two scary truths in the process: First, the hang wasn’t actually Acorn’s fault at all; the second, other apps—among them iA Writer—could suffer the same issue.

Mueller suggested that I check the permissions on a folder I hadn’t heard of before: ~/Library/Application Support/Ubiquity/ . It turns out that Ubiquity is an internal name that Lion uses to refer to iCloud. Mueller learned that occasionally the folder’s permissions become mistakenly tweaked, leaving the folder owned by the root user instead of the logged-in user’s account. When that happens, Mueller discovered, the exact hangs I experienced could occur in a variety of apps—even though those apps don’t (yet) use iCloud.

The easiest way to check that folder’s permissions is to go the Finder, and then choose Go -> Go to Folder. Paste in ~/Library/Application Support and hit Return. Find Ubiquity in that window (bonus hint: type the letter ‘U’ to find it faster), and with that folder selected, choose File -> Get Info. Under the Sharing & Permissions section, make sure that your user account is listed, with Read & Write permissions.

As it turned out, my Ubiquity folder’s permissions were fine, so that didn’t quite explain the hang in my case. At Mueller’s suggestion, I dragged the folder onto my Desktop and restarted my Mac.

Acorn’s hang was fixed. Lion created a new Ubiquity folder, and whatever had gone wrong with my old version vanished.

I can recreate the slowdown by restoring the old Ubiquity folder, or by changing the permissions on the folder with Terminal, and then restarting, and I can see the issue in multiple apps. So if you’ve been plagued by similar hangs after opening files since upgrading to OS X Lion 10.7.2, try this fix. Just make sure to keep your old Ubiquity folder so that you can restore it to its original spot, should something go wrong.