Hi all, I have struggling with this the whole day, but finally found the problem. I will post this so someone else can benefit from this.
The reason for both problems (object not set and AjaxRequest only firing the first time), was cause, due to the fact that I had place my RadAjaxScript-manager not after the <form>-tag, but just before the </form>-tag. So when I moved it to the top, everything started working.
Maybe the Telerik guys could put a note in the documentation about this issue.