Error message: Failed to evaluate expression. Failed to evaluate operator. Failed to evaluate variable.

Hi people

I have been encountering this error message:

Failed to evaluate expression.
- Expression: d(d((-(mod1.chds.nx*mod1.chds.u+mod1.chds.ny*mod1.chds.v+mod1.chds.nz*mod1.chds.w)*mod1.c*test(mod1.c))*(dvol),{test@1}),mod1.c)
Failed to evaluate operator.
- Operator: mean
- Geometry: 1
- Boundary: 42 81 82

I have been working around with boundary conditions, initial values suchlike, but I am not sure if I am on the right track because I either receive the error message above or this error message:

Failed to evaluate expression.
- Expression: mod1.p-mod1.dl.p0
Failed to evaluate variable.
- Variable: mod1.dl.p0
- Defined as: (g const*(mod1.dl.H0*mod1.dl.rho))
Failed to evaluate operator.
- Operator: mean
- Geometry: 1
- Boundary: 42 81 82 83

Why are all these error messages?

Thanks in advanced!

regards
Liwah