As mentioned in my previous post, the photos are too small to see
the actual problem, but it occurred to me later that you might have
fallen into the Pd solder trap. The composition of the only two Pd
solders available from Cookson’s here in UK are:
Easy - 800-850C, 12% Au, 47% Ag and 17% Pd
Med - 870-940C, 89.8% Ag, 4.6% Pd.
If you use Medium, then its nearly all silver, and silver is turned
black by some people.
Could it be that?