I'd like to understand why this happens as well but there has not been a lot of response that I can see. Does nobody know?
No. No-one knows, I think not even IceTV staff, because the PVRs don't send a reason why the error occurs. I also occasionally get the yellow warning when the timer has actually been sent successfully (on Beyonwiz, I don't know whether that happens on Topfield).
The usual reason is that the recording would have created a timer conflict.
When a PVR synchs with IceTV, the PVR sends its timers to IceTV, IceTV works out what timer changes need to be made on the PVR, and sends the changes to the PVR. If the PVR can't set a timer, it sends back an error indication for the timer (apparently without any explanation), and the yellow triangle is set on the Web page to indicate that. The usual reason is that the timer that caused the error conflicts with timers that have already been set.
There's a fairly detailed explanation of how IceTV is thought to work (the actual workings are held as a trade secret by IceTV) on the Topfirld forum. I'd provide you with a link, but Topfield has decided to close the forum to reading by non-members*, so I will not provide links to it. Even though I wrote much of the content of that particular post (it was adapted for Topfields by jpp from a
post about IceTV and Beyonwizes that I wrote on the Beyonwiz forum).
* I don't know why, and I don't much care.