Wish I could help - just wanted to say I empathise with your worry and hope you manage to sort it out quickly. Its odd because they are usually such tough little plants - considering where the naturalised ones self seed, in old walls and disused chimney stacks they probably grow quite well without any tlc or feeding, or overmuch water come to that. I've seen them on dry disused buildings and on shingle islets in rivers so they are surely pretty tolerant. On second thoughts was the horse manure well rotted? If not it may have burned the roots. The yellowing leaves were unlikely to have been due to iron shortage as they don't normally demand an acid soil.

Buddleia (sic!) Lo and Behold Blue Chip is a highly attenuated hybrid that does not retain the resilience of feral B. davidii. Any Buddleja can have a limited life-span in a pot unless taken out and root pruned occasionally with some new compost added. Sickly container plants can also often be rejuvenated by changing some of the compost. It is a shame that you used the pot as a final resting place as that is no longer an option.
One suggestion is that you water with lime-water as the mulching may have turned the pot acid. Simply disperse a little ground garden lime (limestone not slaked lime!) in the water next time - they like lime.
Don't rule out the possibility of long-term root damage from vine weevil grubs or similar pest. Again compost replacement may be the answer.

So glad I saw your answer Buddlejagarden - didn't know the miniature one was different from the rest, or that they were lime lovers - the big ones do fine on our slightly acid soil and seedlings seem to spring up anywhere. This probably explain why they love disused factory walls - it will be the mortar they are enjoying.I've only had my Blue Chip for a year and am very pleased to know this - thank you and thanks to Ronald for asking the question!