The only explanation I have found is listed on the developer's website. The error is called "BLOB has been modified" and the explanation is below (as seen on the website):
Somehow you managed to modify your Pattern Notes, Custom Size Notes or Pattern Stitch Notes, but not save it properly (possibly caused by something like an inadvertent system shutdown). As a result, the pattern data file is inaccessible.
The best prevention of this problem is to always ensure that you have exited the Knitware program properly before shutting down your system.
In addition, you can always recover from this problem if you have routinely backed up your data files.
You can check for known errors on the following link: http://www.knitware.uk.com/index.php?act=viewDoc&docId=9