I have had this issue for quite some time (since 2015.5) however I cannot seem to work out the problem.
I cannot export to a slideshow in 720p or above - the progress simply hangs after about 1-5% and if I stop it and try to start it again, it says there is already a file with that name (clearly there is something that has happened behind the scenes).
I shoot RAW with a Canon 5D MkIII (and always have) and it does not appear to be one particular photo that is causing a problem as I have tried this with many different photos. Most slideshows are anywhere from 40-90 photos in length, with music and a text identity plate. I have also had this problem across two different macOS versions, 10.11 & now 10.12.
I am at my wits end in trying to work this out and am sick of having to settle for 480p video!
Any help / guidance would be appreciated.
There appears to be an issue with LR 6.7 and CC 2015.7 720p and 1080p slideshow output when it contains images that are portrait (vertical) oriented. Please see the solution at this link and add your 'Me-To' vote: