When I start Spaz, the window never appears, or a black rectangle appears and the interface never loads

Your EncryptedLocalStore files, where Spaz stores encrypted data, have likely become corrupted. You will need to delete the ELS files for Spaz.

Note: the Encrypted Local Store is used by all AIR apps, so remove only the Spaz directories.

On OS X

Delete ~/Library/Application Support/Adobe/AIR/ELS/Spaz.AIR.LONGID

On Linux:

Delete ~/.appdata/Adobe/AIR/ELS/Spaz.AIR.LONGID

On Windows XP

Delete C:\Documents and Settings[username]\Application Data\Adobe\AIR\ELS\Spaz.AIR.LONGID

On Windows Vista

Delete C:\Users[username]\AppData\Local\Adobe\AIR\ELS\Spaz.AIR.LONGID or C:\Users[username]\AppData\Roaming\Adobe\AIR\ELS\Spaz.AIR.LONGID