I am having problems with an identical error, except I am trying Union a relatively large dataset (tens of thousands of features) in the middle of some Python code. The funny thing is, when I try the same Union in ArcMap, it completes in 2 minutes. What may be causing this fake "Topoengine" error within my code? The call to Union is completely identical within the code to the operation performed in ArcMap, in that the inputs and params are the same. My theory is that the local RAM is spent, and if so there seems to be no good way to refresh the RAM mid-process. Anyone else experience something like this?