I'm doing the quest Wine Wars: Vermentino and I'm at the point where I've defeated the archeospores (didn't remember they were so difficult to kill...) and I've found a corpse with a letter and a key and the quest log prompts me to look for evidence of sabotage at Coronata vineyards. I went there and I've found a small box buried in what seems to be manure, but I can't open it. Is this a bug? Am I missing something?
Can anybody help me, please?

Thats no bug , you will find on Coronata vineyards altho a person with a letter and key ....but watch out that you neither Coranata Quest nor Vermentino Quest completely finished.

on every vineyard you will find a person dead shurly, with a letter and a key.... and the Quest s will go on

Yes I've even found the evidence, but not where I was searching... after solving the Vermentino problems, I was prompted to speak to Matilda but I started to solve the Coronata problems and there I found the letter. Now I got another quest to investigate on whom the saboteur might be. So I think I did everything right :)