A bit of searching indicates that this is a plug-in error, typically Trapcode Form (or Particular).
While I have do both installed, they are up-to-date versions and the project I was working on used neither of them.
It contained exactly 1 shape layer with 2 groups, one containing a path and the other an ellipse.
The project I had been working on previously did use Particular, so perhaps this has to do with a persistent cache or …?