"This project contains expression errors" but no apparent error!

This must be a bug.

As a test, I created a fresh new project and a new comp. In the comp I added two solids. I made the solids 3D layers, and pickwhipped the Z position of one solid to the Z position of the other solid (see screenshot 1).

Then I turned off the 3D layer switch for both layers and am now getting an expression error warning (see screenshot 2).

That behavior would likely indicate that some of your stuff is generating odd values somewhere halfway through. Similarly it could be that there is a duplicate reference somewhere if you did not properly rename layers, comps and controls when duplicating stuff. Without any info about the exact setup of your project and the expressions usd nobody can give you a definitive answer, though, assuming there's more than the posterizeTime() stuff shown iun your clipped image.

Mylenium

Select all layers and press the 'e' key twice. Somewhere you should see a yellow triangle next to the property with an expression. There must be more expressions somewhere.

You could also have expression errors in nested comps. Clicking on the ^ should expand the warning banner and reveal the name of the layer and problem property. Clicking the magnifying glass should take you to the problem expression.

This is all explained in the User Guide.