upgrade PHP 4.0.1pl2 to 4.0.3 & sessions

Hi, I've just upgraded from 4.0.1pl2 to 4.0.3. After that some code involving objects stored in sessions did no longer work as before. I'm pretty sure it has got something to do with the upgrade since all was back to normal when I put 4.0.1pl2 back. The problem I experienced is as follows: I have a shoppingcart object wich I store as an sessionvariable, containing several products, which are also objects, and it also contains the respective class definitions. If I then want to retreive an item from the shoppingcart, I recreate the productobject in the shoppingcart and return it to whatever script was asking for it. In 4.0.3 I then get errors like "Call to a member function on a non-object" when I try to access one of the objects methods. somehow this worked fine before but doesn't now anymore. I wonder if this problem makes sense to anyone, or if others are experiencing the same kind of troubles? thanks in advance, Wieger Uffink