Hi,
As a side note, if you experience lots of bus hangs and you are using a Zynq Ultrascale sytstem, there is a watchdog that you can enable.
It’s not good for production, but in debug it saves hours of reboots.
Also, to synthesize what Cathalmccabe said: it happens when you access an AXI address where no slave is answering. You must find which program is doing this.
This could a running software that you forgot to stop before uploading a new bitstream.
Also, at boot there is a led-blinking program, and if you are fast enough you could upload a bitstream before it complete and cause the same error. So make sure the leds are done blinking before you do anything or disable it with systemctl stop bootpy.