From time to time the blockchain can become corrupted and cause runtime errors in the QT client. Often times fixing the error is as simple as replacing the blockchain on your computer.
Backup the blockchain from another computer or alternatively you can download a backup here:
v220.127.116.11 and newer: Here
Once you have the files, navigate to your HoboNickels directory in the appdata folder. You can find this folder in Windows by clicking the start button and typing in the search bar "%appdata%". Open the HoboNickels directory.
It is always a good idea to back up your wallet.dat files before making changes to this folder, so make a backup now!
Replace the blk0001.dat and txleveldb.
Start your client and see if replacing the chain fixed your problem.