This is an odd sounding phrase, but I don't think it's incorrect. Out of context as it is, as many Duo sayings are, it seems like a strange phrase. But I would like to know if this was the intended meaning.
"They were some very sunny days." - This has the feeling as if the 'days' spoken of have been referenced already and one might say this in agreement with another speaker or acknowledging the sunny weather in those "days".
"There were some days with a lot of sun." - This has the feeling as if the 'days' spoken of have been referenced already and one might say this to indicate that not all the days were lacking sun.
Can any native speakers comment here as to how these two should be translated or can say how the original would be interpreted with a most likely context assumed?
Yes, I would say "those were days with a lot of sun", or "those were sunny days", but I would probably never says "they were days". It's not grammatically incorrect, but it's a very unnatural sounding English phrase.
"Duo English it is, but human English it is not so much." - Yoda
After reading you comment I agree that it is grammatically correct for English, but as a native speaker it sounds unnatural but only because there is no other context surrounding it. I could see this sentence or one like it written in a book where they are referencing a memory or referring to a specific time or location.
I had to think hard about this one, but I came to the same conclusion. If we're talking about a group of "days" that had already been referenced, we could say that "they were sunny days".
My initial assumption was that not all of the days we're talking about were sunny, but that's not necessarily what the sentence means.
Them were some days with a lot of sun. Seems ok to me. And translates well also I think. Not accepted though?