Error with expression after pre-comp

When I started Ae and opened up the file this morning, the error had disappeared and everything is working as it should. I think the "random bug" diagnosis was correct.

I have seen this happen with expressions that look at another layer based on time. It happens once in a while when scrubbing but if you preview or render the error goes away. I think it's just a random 'what time is it really' a bug that pops up occasionally when expressions point to properties in another comp.

This seems like a bug. The strange-looking icon that appears in the Error Message is something I too have seen. Please file a Bug Report and include AEP if you can. This bug appears intermittently and this makes it more difficult to nail down. 

Maybe the issue originally come from the expression that you're using on the Null Rotation so it affects the expression on the text layer, so let us know what expression you use on the null layer 

Have you actually expanded the little arrow in the warning for more details?

Mylenium