That's only assuming that in John Jesus really meant "born from above" not "born again." This doesn't appear to be the case, however. Whether Jesus was speaking aramaic, hebrew, latin, or german, the metaphor he was using could easily be misunderstood simply because of it's metaphorical nature.
In other words, Ehrman's argument runs as follows:
1) Jesus spoke aramaic
2) In the relevent text, he uses a greek word with two meanings
3) Jesus means "from above" (one meaning) but nicodemus interprets it as "again" (another meaning).
4) This misunderstanding would not have occured in Aramaic, where no word both means "again" and "from above" and therefore either the conversation was in greek (unlikely) or didn't happen.
The issue, however, is that premise 3 doesn't appear to be correct. The issue wasn't with using one of two meanings but using one meaning metaphorically. In this case, the aramaic would have had the same issue.
I think you misunderstand me. Let's look at translations for a minute. In English, there are seperate words for "word" or "story" or "account" or "narrative." In Greek, all of these could be translated as logos or mythos. If I were translating from English to greek, it is easy to imagine a situation where I meant "listen to my word" not "listen to my story" and this distingtion was lost in the greek translation, which might be rendered both times into the same sentence: akoue ton mython mou
The point is, just because the translation has a certain ambiguity doesn't mean anything unless the ambiguity is part of the narrative. If Jesus had said a word in greek with two meanings, and Nicodemus understood one of them when Jesus meant another, this would matter. If Jesus uses a word which just happens to mean to things, but the misunderstanding results from something else (metaphor), then this only matters if this metaphor is impossible in Aramaic. It isn't.