|Haven't run into it and assume you mean an update to SpyBot's immunization feature and/or a bug fix.|
Maybe worth uninstalling the whole thing, rebooting, then downloading and installing SpyBot again. There's always the odd chance of an update going awry, although SpyBot has had a good record.
You could also, as previously suggested, avoid the immunization feature and use SpywareBlaster instead.
Sorry can't think of much more, maybe someone else will have some ideas.