Hello,
have you tried to disable CDP on cisco switch port?
Does the HP switch sends STP BPDUs and in what format?
I would try use a sniffer to compare what is received on cisco switch port and HP switch port.
However, CDP messages are multicast and should be propagated by HP switch.
the POE should be able to detect a device that doesn't need in line power or the NIC of the server could be electricallydamaged.
Also verify if port is stable on cisco switch checking logs with
sh log
Hope to help
Giuseppe