Errors in constant requests to CRM

Hi All,

We have a web service which is connected to the CRM. Requests to the web service .. thus also to the CRM are being done constantly every minute for multiple devices. The problem is that after some time running, errors like (cannot amend closed or cancelled activity) and others are being invoked thus failing the request.

Do you know what could be causing this please? Is it some kind of memory leak? How can we solve this?

Thanks a lot in advance!