What did actually happen during Locke's first visit in the cabin?

It was actually later revealed by Ilana that the person in the cabin was not Jacob.

Exiting the cabin, Ilana informed Bram that Jacob was not there, that he had not been at the cabin in a long time and that someone else was using it. She then ordered the others to set fire to the cabin. The group departed, leaving the structure to burn.

She further states that he hasn't been there in a long time and that someone else has been using it, therefore her desire to burn it to the ground. This is proven when on Locke's third visit to the Cabin, Christian Shepard is there with Claire. At this point we know that MiB is Shepard's father on the island (his ability to take on the forms of the dead), which would imply that he is the one now using the Cabin.

It is heavily implied that this someone is indeed The Man in Black (MiB) impersonating Jacob. It is not a surprise that MiB has been manipulating people just as much as Jacob in order to win his side of the Game between the two of them.

I believe Ben says this is Jacob because it is later revealed that Ben has never seen Jacob until the night MiB manipulated him into killing Jacob. So he would say this is Jacob because he saw the strange occurrences with Locke on that first visit. His bewilderment and expectation that no one would actually be there would lead him to believe that it was Jacob when it was not.

It should be worth noting that the producers/writers have not explicitly stated that it was indeed MiB inside the cabin and they have yet to reveal this at all, nor I don't think they ever will. But all the episodes involving the Cabin and what we know of MiB points to the extreme possibility of him being the one that Locke and Ben saw inside the Cabin.

For me, it's clear that it was indeed MiB. We see the mysterious man in there when Ben and Locke make their first visit, along with a short shot of "the eye". The next time we see the cabin, we see Christian Shepard in there along with "the eye", this is in S4E1 when Hurley runs into the moving cabin. We know from S6E13 that MiB was taking the form of Christian Shepard, and since we also see "the eye", that makes the link with Ben and Locke's visit. Of course, near the end of Season 4 when Locke goes in alone, it's Christian Shepard in there with Claire, who later talked about MiB impersonating her father, making the connection of MiB with the cabin even stronger. In S5 MiB makes Ben admit that he never met Jacob, and that he was "as confused as Locke" about what happened in the cabin. Finally, Ilana and her team go there and she concludes that Jacob "hasn't been there in a long time", the ash circle (which we know keeps the MiB out from S6) has been broken, and "somebody else has been using it".

I would say all the clues are there that we only ever saw MiB in the cabin, although apparently Jacob did use it at some point, with the ash circle in tact to keep MiB away.

It's worth noting that the primary thing MiB uses to get Ben to kill Jacob (and it's easy to forget this because later the focus is on Alex) is Ben's jealousy over his sense that the island, i.e. Jacob, prefers Locke to him. I think that MiB gave his "help me" message only to Locke as an early step in making Ben jealous of Locke, eventually leading Ben to murder Locke, only to have MiB take Locke's form - making Ben even more jealous thinking the island has even resurrected the "chosen" Locke. Then, "Locke" demands to see Jacob and is immediately taken there, while Ben was denied, leading Ben to kill Jacob.