Did anyone try to flash from the TP-Link GUI directly, or via TFTP? If via the GUI it might be the filename that is causing the problem. In the installation link you posted there is this note that the TL-WR841N firmware does not accept the OpenWRT filename. Maybe the TL-WR802 firmware is doing the same thing?
":!: Note: Newest version of tp link firmware (tested on TL-WR841N_V9_150310 / 3.16.9) does not accept openwrt firmware filename,
so you have to rename the downloaded openwrt firmware to something what looks
like stock firmware prior flashing,
i.e. wr841nv9_wr841ndv9_en_3_16_9_up(150310).bin
for V9 - check the zip with original firmware on tp link website for correct file name scheme - I'd suggest to remove "boot" word from filename, because openwrt firmware does not contain boot loader."
https://forum.openwrt.org/viewtopic.php?id=46475
