checksum
Some component of your Axim ran an algorithm to compare what it thought should be where it's looking with what actually was there, and found that the
checksums did not match.
If you've tried a Soft Reset and it still doesn't work, try a Hard Reset
using the buttons, this should provide a pop-up asking if you are sure.
Press Cancel and it
might reboot correctly. If it doesn't, you must actually do the Hard Reset by answering Yes in the pop-up. Or by removing BOTH batteries - X5 ONLY, (or just letting them drain to empty - X3 & X5). As soon as power is zero, the unit will automatically have Hard Reset as there is no power to keep the RAM alive. No pop-up screen with this method.
Hope you have a backup to restore to after the Hard Reset, otherwise you will have to reload all your apps.