The _libs folder is only relevant for those working with the source code.
My guess is that you did not manage to upload the Brettle.Web.NeatHtml.dll, you may have one on your server that it didn't overwrite, the incorrect (older) version is 36kb and the correct one is 32kb in size. the correct one was included in 126.96.36.199.
However, version 188.8.131.52 is now available from the download page.
Hope it helps,