As the title says; emerge-webrsync tries to invoke tarsync directly against an lzma file. It doesn't support lzma natively, thus it will always fail.
This is fixed in git:
The problem is people may get stuck (as I did), without the possibility to sync again. Thus they won't get the fix.
A possible workaround is to unmerge tarsync, run emerge-webrsync, and the emerge tarsync again.
It would be wise to publish this issue somewhere in Gentoo (forums ?), so people become aware of if.
This is in 2.2_rc68, but I'll leave this bug open until it's in an unmasked version.
This is fixed in 2.1.9.