Hi,
I have been getting the same problems as mentioned on these post.
However, I have managed to fix the problem!!!
I removed the PoE hat from the Raspberry Pi.
Used jumpers to connect the 4pin PoE header on the Pi to the 4pin header on the PoE hat.
Stripped a micro usb cable, crimped Dupont connectors to the red and black wires. Connected these wires to the 5v and GND on the PoE hat. Plugged the micro usb connector end into the micro usb connector (power) on the Raspberry Pi. I then connected a flash drive, PoE Ethernet cable, monitor etc... to the Raspberry Pi. The Raspberry Pi booted without any errors. I then ran numerous large file copies from the Raspberry Pi to my PC over the network, without any issues.
I didn't connect any of the 40pin GPIO headers from the Raspberry Pi to the PoE hat so no I2C connections. I also noticed the the link speed indicator on my hub changed from yellow (10/100mb/s) to green (1000mb/s).
I did read somewhere (can't remember where) that the Raspberry Pi shouldn't be powered via the GPIO.

- 20180904_124433_001.jpg (197.51 KiB) Viewed 1768 times
Anyway this seems to work.
Have fun.