I had an issue last night with my Archer C7 that I wanted to post about. I bought my Archer C7 off Amazon 27 November 2015, and it shipped with a different version of the firmware than was up on TP-Link's website:
Firmware Version: 3.14.3 Build 151014 Rel.49676n
Hardware Version: Archer C7 v2 00000000
I downloaded the correct factory.bin for the hardware version I had, checked the MD5 hash, and attempted to install OpenWRT. I got the following error again and again:
Error Code: 18005
Upgrade unsuccessfully because the version of the upgraded file was incorrect. Please check file name.
I also tried to "update" the firmware to the previous "Archer C7_V2_150427" firmware, and I got the same message. After some digging around, I found this thread (http://www.dd-wrt.com/phpBB2/viewtopic. … a995bc5d10), where OP had a similar issue. The folks at DD-WRT put up a new version of the old 150427 firmware that could be flashed on top of 3.14.3 Build 151014 Rel.49676n (you do need to be logged in in order to see the attached file. I also mirrored it here if that's okay: http://www81.zippyshare.com/v/r3oZQK2R/file.html). I successfully flashed to this version of the TP-Link firmware.
I was then able to flash to the current 15.05 CC version of OpenWRT. Very happy with the performance! I hope this helps someone else.