I had a look at the download section and saw, that the server update iso 7.2 is not available if you select esxi 5 as operating system. if you switch to server 2008 r2 the new version is in the listing.
at the esxi 5 listing, there is server update iso 7.1.1 avalable. this is the one i used with the r515 which are running without problems.
i think i will downgrade the new machines to server updates 7.1.1 and try ...