It means your copy of the blockchain is corrupt and needs replacing.
Go into your XP data directory:
Windows: %APPDATA%\eXperiencePoints\
Linux: ~/.eXperiencePoints/
Mac OSX: $HOME/Library/Application Support/eXperiencePoints/
Delete the following folders:
blocks
chainstate
database
sporks
zerocoin
Download the latest bootstrap and extract it (see “How do I use the bootstrap/snapshot?” for details).
Restart the wallet and let it sync fully.