Web Service invoke error after severall calls

Hello experts,

We are facing an invoke error problem regarding a customer Web Servcie.

It works fine at the first call, but when we keep on calling de WS, it finally fails at the 30nth or 20th call (for example), i.e: when multiple calls have been made one stright another.

Here is an screen shot of the error:

Thanks in advance for any help provided.

J