2D->3D

Hi Everyone,
I successfully build a model for a redox flow battery in 2D, but now I tried to convert it to 3D and following error occurs. I have read the others threads regarding this error but couldn't find a solution. There it is said that it could be a missing BC but since it works in 2D this can't really be the case in my model, can it?

This is the error message:

Failed to evaluate variable Jacobian.
- Variable: mod1.phil_neg
- Geometry: 1
- Domain: 1
Failed to evaluate operator Jacobian.
- Operator: mean
- Geometry: 1
- Boundary: 6
Failed to evaluate variable Jacobian.
- Variable: mod1.phil_neg
- Geometry: 1
- Boundary: 6

Thanks for your help.
Michael