LIGHTROOM Image won't export to Photoshop when tagged with a colour label.

Do you have Nik plug-ins installed? That is usually the issue with Edit In not working.

No I don't use Nik. I was able to narrow the problem down to that one specific thing. If I choose a colour label, then it will not export. Works fine until I've tagged one of my photos with a colour label.

I was hoping the green-label was merely a contributing factor in conjunction with something else that is known to cause the problem, Nik plug-ins.

The terms Edit In and Export are both used in your original description, and despite being different ways to open photos from LR to PS, both work for me with LR 5.3 and PS-CC 14.2.1/ACR 8.4 RC on Windows 8.1 with a green label set. Since I am on Windows, I am unsure what "Cmd-E Apple" corresponds to.

Specifically:

I can set a green label on a raw photo and use Ctrl-E or right-click Edit In PS, and the photo comes up in PS-CC without creating an intermediate file on the hard-disk.

Or I can select one or more photos (that also have a green label assigned) and use File / Export or Ctrl-Shift-E to start an Export operation and have Open in Photoshop set as my Export Post Processing Action and those photos are exported to my hard drive then all opened in PS.

So what is occurring to you is not a universal bug where a green label prevents using Photoshop from LR in all situations, though that does seem to be a determining factor for you.

Can you create a new catalog with a single photo imported that has no other adjustments other than having a green label assigned and that has the problem? If so then it might be worth having Adobe or others try that exact catalog and photo on their systems to see if the bug occurs for them.

Are you updated to the latest OS 10.9.1 and PS-CC 14.2.1 and ACR 8.3 (or ACR 8.4 RC) and LR 5.3 or are you running older versions of some of those software components?

Interesting Idea! I will try creating a new catalog, and import 1 photo and try that.
I'm using a Mac, the shortcut is using the Command key with E, on a pc that would be CTRL-E.
I do have the latest version of Lightroom running, but have to say that I never had this issue in Lightroom 4, and this issue occurred as soon as I upgraded.

LR5 has new minimum OS version requirement so it may be using different OS-level functions for the keyboard and starting up external programs so there could be different things that affect LR5 compared to LR4.

Are you also up-to-date with your OS (10.9.2) and PS (14.2.1/ACR 8.3)?

Are these JPGs or raw files?