The yellow triangle usually means that IceTV has tried to set up a timer to record the show on the PVR, but the PVR has reported that the timer could not be set. The most usual reason for this is that there are more simultaneous recordings being set than the PVR allows.
I understand that the PIMP protocol used to communicate between the IceTV server and the PVR is able to carry error information about why the operation failed, but Beyonwiz PVRs, at least, do not supply any information in this field. If it is available, this information is displayed in the IceTV web page entries for the shows in TV Guide and My Week.
I don't have any experience with Humax PVRs, and I don't know whether they provide any useful error information back to IceTV when a timer can't be set.