1. What happened

The issue comes from trying to upload a project that was unlinked and relinked within the same day. After creating a new project and enabling Collab I uploaded 15k assets successfully using 2017.2. I then unlinked, relinked the project in 2017.3 and attempted to upload my 15k assets (including .meta files) again. However, my upload fails after the first batch consistently with the following error message: "Cannot submit change to server." In some cases, this results in the first batch succeeding but the snapshot doesn't get updated to indicate that it succeeded. So there is a pending Update from the server that consists of the same files that were just uploaded. The best workaround is to avoid batch uploading and try partial publish to upload smaller selections of assets.

2. How we can reproduce it using the example you attached

a) Open the attached project with 15k assets in 2017.2.
b) Enable Collab.
c) Publish all assets using the Collab Toolbar.
d) Unlink the project, open it in 2017.3 and relink the project.
e) Enable Collab and Publish all assets again.
f) Wait for publish to fail (repeat if necessary).
g) Find the "Cannot Submit Change to Server" error in the Collab Toolbar and Editor log.

Reproduces on 2017.3.0f3 and 2018.1.0b3.
Does NOT reproduce in 2017.1.1f1 or 2017.2.0f3.