[Zope] May be a problem with ZClass

Hello everybody. My problem: I made a product with ZClass technology. I don't know why every time that I start Zope, an old version of my product appear. The unique information that I could find is in the file "stupid_log_file.log". This is: 2002-05-27T14:01:46 PROBLEM(100) ZODB Could not import class 'MixGraphClass' from module '*Z70QZ0TZA5T3p/X/EFDKrA==' Before this problem appear the product work fine, and if I re-import the newest version after a start, the product work fine too. So I don't have any idea of what is happening and how I could solve this situation. I didn't find anything in the newest version that didn't be in the older version that suggest me a "code error". Can someone give me any suggestion? Thanks, Martin