an error occurred while exporting (error 5).

Trying to "share" a 24 minute 3840x2160 29.97p project. It has failed multiple times. I get the following dialog:

The share operation “Master File” has failed.

The operation could not be completed because an error occurred while exporting (error 5).

I have done other projects with same settings and various lengths of 28 to 32 minutes with no problems

I have, rebooted. Deleted all render files. Created new project and placed clips in project.

What is an error 5?

iMac (Retina 5K, 27-inch, Late 2015)

4 GHz Intel Core i7

Memory 40 GB 1867 MHz DDR3

Graphics AMD Radeon R9 M395X 4096 MB