please force aria2c to overwrite files by default (will not work with repoman otherwise because it tries to download metadata.xsd when an empty one is found, but aria2c will not overwrite it)
If you get to give aria's --allow-overwrite=true a shot before me, please let me know if that fixes the issue.
Fixed by commit: https://github.com/gentoo/fetchcommandwrapper/commit/b942b007aa1fba72226076ba443b0f4e08d16069 Version bump in Gentoo: https://github.com/gentoo/gentoo/commit/3ad5dff4a9d5b9d8cb34c04471be80e7f83505e0