No order status update from “on hold” to “processing” if payed with PayPal

Good evening,
I have an issue with PayPal IPN on my website site: I am using the PayPal standard integration and everything is ok for all orders except the ones that contain a bundle product realised with WPC Bundle.
Orders that not contain the bundle product are payed and the order status is automatically modify in processing and, on the top of the backend order page I can see the transaction number.
If an order contains a bundle product and the customer pays with PayPal, also if the payment is ok and I received money on my PayPal account, the order doesn’t change its state and it remains “on hold”. On the backend order page no transaction number appears.
If I check the IPN message sent by my PayPal account I notice that the messages referring to orders contain bundle product are not sent and che error code is 500.
It is possible that this is caused by the IPN script that for example is too long? Or I notice that, in the script, there is the item name that in the case of bundle is very long and so it is cut at the end (there is the name of the bundle followed by the names of products that compose the bundle, but at a certain point it is cut and there are …. ). Can the cause of my issue is the item name?
How can I modify the IPN script? Or what should I try to solve the issue?
Any help would be appreciated!
Thank you very much
Federica