It is a little hard to tell exactly why RORB is crashing, but I have encountered something similar to this before where RORB has trouble interpolating when the discharge is barely changing with increases in storage.
While this is a little clunky, what you can do is fudge the storage discharge relationship such that no water is exiting the storage at all and then insert an outflow/diversion upstream of the storage. With the diversion you specify a relationship between the water coming in and the water being “diverted”. This diversion you can set to 0.3 m³/s to simulate the pump. You can then set an inflow downstream of the storage so the pumped/diverted water then comes in downstream of storage. I believe doing this should get around the interpolation error. Let me know if you are still having problems.