PerfectJob Software 905 Fatal Error

Hello guys,
Please help me find a solution to my problem. I downloaded and installed the PerfectJob Software trial version. When I start the software, it shows a flash screen showing “Version 1.0.4.96…. Updating Database Schema…” and then gives me the error message written here below.
The headline is: "905 Fatal Error "
The message itself says: "5074 The Index 'Unique_Username' is dependent on column 'Username'. ALTER TABLE COLIMN Username failed because one or more objects access this column. Command: /****** Version: 1.0.1.13 ******/ ALTER TABLE [dbo]. [User} ALTER COLUMN Username vachar (255) null; "
Have you seen this kind of error message before? What does this mean?
Help me out please.
Thank you.
