Why is this dimension red?

[etfrench]

There are no warning or error messages.  Changing one of the driving dimensions will turn it black.

[ltomuta]

What is the expression of that dimension?

[etfrench]

It just equals the driving dimension above it.

[ltomuta]

Indeed, I downloaded your design and saw it. But the dimension is not red
in it while all the values seem to be the same. So it seems to be something
else that bad math in an expression as I was initially suspecting.

[davebYYPCU]

The inner circle is driving dimension?  Looks like a driven.

More likely a tolerance thing where the fifth dimension in the chain, breaks its calculator.

Might help….

[etfrench]

Adding the driven dimension of the inner circle turned the offset dimension red.  Adding a driven dimension to other circles will also turn their original locking dimensions red.