Active/Inactive mesh nodes and undefined initial conditions

I've got some nodes which have been made inactive, and I'd like to use them again.

I get an error suggesting "Active mesh nodes with undefined initial conditons are inhibiting the simulation start"

How can I find exactly what hasn't been defined? I've reset everything obvious.

I assume, you made a simulation with inactive elements and used the computational findings of the process variable as an initial condition for a second run. The second run has either other inactive elements or no inactive elements at all. If you do not re-initialize the nodes, which are completely connected with inactive elements of the first run, you start your simulation with NaN (Not a Number). This is not possible in a mathematical sense. Accordingly, I suggest to re-initialize the process variables before you run your second simulation.