Error reversing flow in multilayer wells

Hi,

I am trying to run a model with seasonal geothermal storage and therefore need to change the flow direction in my existing wells at certain times.
However, when changing flow I get the standard error:

PCG has diagonal element <=0.

I know this is usually a discretization error or too big contrasts in material properties in neighboring layers/elements.
I have tried to refine the model both vertically and horizontally and changed the reservoir parameters to be homogenenous, but none of these solutions helped.
Also played around with the time-step length, but that didn't seem to have any influence either.
Worth noting that I use a few days of zero flow rate before I reverse to pumping/injection respectively.

Could the dip in my reservoir layers have an effect here?

I have done similar simulations before on simple box models and had no issues.

Any ideas would be much appreciated!

Hard to judge... you could try - in addition to the zero pumping phase - to add a ramp to the pumping/injection time series. However, I'd typically also link this error more with spatial discretization than temporal behaviour.