First of all I created a issue, then I added a note and a monitor.
When I want to update a field of an issue via SOAP, there are lots of error messages.

In the File "bugnote_api.php" I found the answer. If you look at the function "bugnote_get_all_bugnotes" you will see there is a array called "$t_bugnotes". On the picture "solved.png" you can see below you can find a answer to the problem, you just have to add the marked line.