  We investigate optical frequency transfer over a 1400~km loop of underground
fibre connecting Braunschweig and Strasbourg. Largely autonomous fibre
Brillouin amplifiers (FBA) are the only means of intermediate amplification,
allowing phase-continuous measurements over periods up to several days. Over a
measurement period of about three weeks we find a weighted mean of the
transferred frequency's fractional offset of $(1.1\pm0.4)\times10^{-20}$. In
the best case we find an instability of $6.9\times10^{-21}$ and a fractional
frequency offset of $4.4\times10^{-21}$ at an averaging time of around
30~000~s. These results represent an upper limit for the achievable uncertainty
over 1400 km when using a chain of remote Brillouin amplifiers, and allow us to
investigate systematic effects at the $10^{-20}$-level.
