Manchurian Bush warbler or whatever their new name now have a more square ended tail where Brownish-flanked BW have this very rounded rectrices tip with a forked tail impression. Tail looks also very thin at the base, as other cettia due to less quantity of rectrices than other passerines I guess. I'm quite confident on Brownish-flanked BW but I'm very happy to be corrected.
All the best,
Jonathan