Uploading large project does not work

[Stereo Missile]

Ok so I have an existing session which is a full song, quite a lot of tracks and a fair sized session (about 2GB uncompressed).

So I have tried now 3 times to "Save Copy In" a new project, exporting the files as 24 bit 44.1k wav files. At first it seems to work, but then in the task manager it just gets stuck. It uploads some of the project but it never succeeds with the whole thing. Have tried signing out etc. I have a premium cloud account (which is useless because the upload doesn't work properly!), and plenty of free space in my cloud account.

I also have very fast wireless internet (100Mb/sec down, 40Mb/sec up). In all 3 cases this process has got most of the way through uploading but just is never able to complete properly and is unusable by the other collaborator. I have left it for hours each time, logged out and in again, tried everything. My fast internet costs me about $10/GB so I have already spent $30 on trying to upload this to no avail.

Then after much frustration I exported it as a session, compressed using Source-zip to a zip file (852 Mb), and put it in my dropbox. With dropbox over the same internet connection, it uploaded successfully in 6 minutes.

I have all the latest updates, top spec Mac Pro. So what is the problem? Are other people experiencing this? I have previously been able to upload small test projects successfully and shared them successfully on this system, but with an actual real world session of a reasonable size it just doesn't work.

Is anybody else experiencing this?

I'm on:

Mac Pro 6,1 12 core, 64G Ram, OS 10.10.5, Protools 12.6.