I've found a similar question (for the same printer) on another website. The solution that worked in that case is copied and pasted (complete with the original typos!) here:
1. Press X power at the same time 2. Press BLUE, GREEN, GRAY 3. press BLUE until you see "INFORMATION MENU" 4. press OK 5. Press Blue until you see "CHECKSUM FOR RELOCK DATA INPUT" 6. OK 7. Press X until you exit all menius 8. When promted for aliement cancel it with X. It should bypass the error
If you can make sense of that lot, it might be worth giving it a try.