Looking for negative reference to women in the Avatamsaka Sutra
I downloaded the Chinese translation of the Avataṃsaka Sūtra and did a few searches for patterns with different combinations of words that are likely to appear there (e.g. 女 "woman", 地獄 "hell", 種 "seed", 夜叉 "Yaksha"). I found quite a few phrases, but none of them looked like the cited passage. My knowledge of classical Chinese is very rudimentary, so I may be mistaken. However, the search inside the Cleary translation on the Amazon website doesn't show any such passage either. So if it is there at all, I think it's more likely that the quote is a summary/paraphrase of a larger chunk of the sutra, and not a direct translation.

EDIT: I found a classical Chinese/classical Japanese phrase that is the source of your quote:


  女人地獄使。能断仏種子。外面似菩薩。内心如夜叉。


It can be found in this text at 7-094. Unfortunately, there is no information about the author nor the date when it was written. In any case, it is definitely not modern Japanese, so I don't think it was written recently. It seems to be a Nichiren commentary to the Lotus Sutra.

EDIT 2: The above quote is preceded by "華厳経云", which means "Avataṃsaka Sūtra says". That explains why the passage is commonly (and incorrectly, as we may conclude) attributed to this sutra.

EDIT 3: I found information about the origin of the quote, which, in light of the facts presented above, we can regard as conclusive:


  This statement is not found in the extant Chinese versions of the Flower Garland Sutra. However, A Collection of Treasures written by Taira no Yasuyori during the Jisho era (1177–1181) cites it as a quotation from the Flower Garland Sutra.