Error messages starting with Assertion failed: followed by offset[i] > offset[i-1] or handle[n] != 0 hint at an attempt to retrieve the same field more than once, e.g. by specifying param=z/t/z or step=24/48/24. The actual message varies depending on how exactly the data is retrieved.
The general advice in such cases is to check the request for multiple occurrences of keyword values. see How to report a Web API issue - Web API FAQ.