Yes - it happens both ways.
The first time it happened on the MAC - I saw that all my sprites were missing their Material references, so I opened each Sprite Collection and re-committed it. (Pressed the Commit button).
That fixed all of the problems on the MAC. Then I checked-in all those files and when I synced to the PC and opened the project I got the exact same errors. Opening each Sprite Collection and re-committing once again fixed the problem, but obviously this isn't a viable solution.
My project is set for 2 platforms (1x and 2x). I did notice that in the 2D ToolKit preferences dialog, under platform - nothing was selected, so I selected 1x.
Also as a rule of thumb, when I add a project to Perforce, I add the entire Assets directory. After reading a piece about using 2D Toolkit and version control - I saw that there were many files used by 2D Toolkit, that should NOT be added to version control (i.e. -tk2d.asset, -tk2d.asset.meta and all of the files in the Resources/tk2d directory).
I removed all of these files from version control, deleted the library directory, opened Unity and rebuilt the 2D Toolkit indexes. This was on the PC. Then I checked-in, checked-out on the MAC and did the same - no help. All sprites are missing their material references.
I'm at a loss for what to try next.