Highlighting the issue #840 , tof provided by the user ( 2 hr or 7200.0 secs ) exceeds the upper bound tof( 1989.6321243010739 secs ) for the given r0,rf.

tof_repeats.py

from poliastro.iod.vallado import lambert as vallado
from astropy import units as u, constants as c


if __name__ == "__main__":
    k = (c.GM_earth.to(u.km ** 3/ u.s ** 2))
    r0 = [10000., 0, 0] * u.km
    rf = [8000., -5000, 0] * u.km
    tof = 2 * u.hour
    numiter = 10**3
    rtol = 1e-6
    print("\nLong\n")
    v_long = vallado(k, r0, rf, tof, short=False, numiter=numiter, rtol=rtol) 
    print(next(v_long))
    print("\nShort\n")
    v_short = vallado(k, r0, rf, tof, short=True, numiter=numiter, rtol=rtol) 
    print(next(v_short))