When exporting a mix of photos and videos, Lightroom 6.10 processes the first few and then just stops processing any more. Progress bar still appears, but no matter how long you wait, nothing more happens. Activity Monitor shows activity while LR exports the first few, but then drops down to zero again. I have just spent 3 hours validating this with a support engineer (case #0188969079) on multiple Macs. The support engineer saw this first hand on both machines. One is an older Mac, one is brand new, with a brand new OS installation, and a fresh LR 6.10 installation. The only solution was to downgrade to 6.9, which then worked perfectly again. I checked 6.10 on a Windows machine, and that works fine. After spending all the time with the engineer, I was told that the only way forward was to report the defect here.
I have also tried upgrading one of the Macs to 6.10.1, but no luck - Lightroom export falls over after it's done a couple of the videos. It always seems to start with the videos, during which it fails. But never half way through a video. Always fully completes a video before it then fails.
And final comment: if I export each of the files individually (or even in smaller groups), that works fine in both 6.9 and 6.10, for both photos and videos. It's when I select all files in a collection at once that LR fails (I've tried a collection with 10 files and one with 30 files, both with the same result).
@Simon: Yes works fine if it's only photos. Also seems to work fine if it's only videos. It's when it's the combination, and when I've selected all items in a collection that it fails, even as little as 10 items. I'm using the same video export setting I've always used with past versions - H.264 High.
@Simon Unfortunately they are all personal family videos/photos, but will try grab some that are not. I now seem to have 6.9 working reliably but not 6.10. I've bee trying to strip all settings/plugins etc out and see what I can discover. My current hunch is that it might be related to a plugin that is incompatible with 6.10 for some reason. I have been using LR/Mogrify during post processing to tag photos with date during export. This has always worked fine in previous versions (including 6.9 today) but looks like it may be causing issues in 6.10. Diagnosis continues...
This is still an ongoing issue, but I did find a workaround that is interesting. After I perform the export, and it fails after completing some of the video files, I simply run the export again immediately, and this time it works fine. Can't even begin to explain that one, but at least it's a workaround. This is on 6.10.1.