Unexpected EOGeneralAdaptorException

Hello guys,
I'm quite confused about an issue I'm having, which lead to:
------
Error:
com.webobjects.eoaccess.EOGeneralAdaptorException: Unable to convert  
value of class java.lang.String for attribute 'deletedOn' in entity  
'Member' to adaptor type EOAttribute.AdaptorBytesType. Check the  
signature of the conversion method  
com.webobjects.foundation.NSTimestamp.toString().
Reason:
Unable to convert value of class java.lang.String for attribute  
'deletedOn' in entity 'Member' to adaptor type  
EOAttribute.AdaptorBytesType. Check the signature of the conversion  
method com.webobjects.foundation.NSTimestamp.toString().
------

But the "deletedOn" attribute is a NSTimestamp in the EOModel (and in  
it's custom java Class), a DateTime in the datastore (MySQL)...
But more than this I have other attributes (like a dateUpdated one)  
which are exactly the same and don't lead to this crash:(

Does anyone have an idea on where it may come from or how to debug that?

Thanks for your help,

Xavier