One user can't open anything after Yosemite upgrade My Mac mini (early 2009) was getting slow, so I decided to do some upgrades. I got 8 GB of ram and a 128 GB SSD from crucial that their wizard verified as being compatible. I downloaded Yosemite from the app store and made a bootable USB drive following Apple's instructions. Then I did a time machine backup to an external drive. Finally I did the RAM and SSD install. I inserted the Yosemite USB drive I made earlier and did a fresh installation. On startup, I migrated files and settings from my time machine backup. I logged in to my primary account and did the software updates from the app store and rebooted. At this point my family started using the computer and is very happy with it. Startup, login, and application loads are very fast. There's no more freezes or beachballs.  

However, one of the four user accounts is problematic. It takes several minutes to login and load the dock/ menu bar, versus seconds for all the other accounts. Clicking on anything in the menus/desktop/dock/dialogs results in a beachball for several minutes. No applications or files will open. Occasionally I'll get a "the application ... could not be opened" error dialog 10-20 minutes after trying to open something from the dock. How do I fix this account?  

Some additional information: this computer does not have filevault encryption enabled, and there are no login items.
There is likely a corrupt preferences or other file in the user's ~/Library folder. Honestly your best bet is to create another users folder for that person, copy their files over and set up Mail, Messages, Safari and the like from scratch.

Once you have a working profile you can slowly migrate other things from the old to the new user folder (old Mail folders, Safari bookmarks, etc.) till you have enough restored for everything to work as necessary.

At that point you can delete the old user.