This was first reported in #6446, although #6446 is actually two related issues - this one and a slightly different one (which I am declaring #6446 to actually be about).

This issue is that differential objects can't get written out to ECSV. To see this in operation:

import io
from astropy.coordinates import SkyCoord
from astropy.table import Table
sc = SkyCoord([1], [2], unit='deg', galcen_v_sun=None)
t = Table([sc])
t.write(io.StringIO(), format='ascii.ecsv', overwrite=True)

This raises a RepresenterError: cannot represent an object: ( 11.1, 232.24, 7.25) km / s

(if the galcen_v_sun=None business isn't obvious, it's that it sets that attribute to the default value:)

>>> sc1 = SkyCoord([1], [2], unit='deg')
>>> sc1.galcen_v_sun
>>> sc2 = SkyCoord([1], [2], unit='deg', galcen_v_sun=None)
>>> sc2.galcen_v_sun
<CartesianDifferential (d_x, d_y, d_z) in km / s
    ( 11.1,  232.24,  7.25)>

So the core problem seems to be that CartesianDifferential isn't something that ECSV can handle. @taldcroft, is this because these features were being implemented in parallel and therefore it's an oversight? CartesianDifferential is very similar to CartesianRepresentation, so I'd think it would be pretty straightforward to include?

cc @larrybradley @adrn