I had a similar problem and managed to solve by changing the header:
If-Match : 1411063473041
If-Match : "1411063473041"
I use a REST Client in Firefox.
For anyone who, like me, started having the 412 errors in NSX 6.4.5 when using PUT requests - in this version the quotes have to be removed from the header parameter, or else, if you pull the etag number and pass it on without editing, your REST call will fail.
Status code: 200
Status code: 412