Well, I’ll be frank, I’ve not come across an error like this on the Leetro MPC6515 controller, in fact I didn’t know that the controller even had inputs for water protection (and I’ve not been able to find a pin-out that says it does, though I’ve not looked exhaustively).
On a HPC 1290 the flow sensor is on the right side behind a panel below the tube. It looks like this:
If you’ve had the cutter apart (say in half to move it or similar) this is a component that can be over looked at reconnection, though again, as mentioned, I’d expect this to send a signal to the high voltage PSU rather than the controller board.
I’ve also seen some HPC1290s with a bank of relays in the electronics bay on the left side AND with the controller board exposed to the elements. Check all this looks correct, no loose connections or shorts.
Other than the error message (I’d love a photo of it) does everything work as it should? Does the laser fire? Does your High Voltage PSU have an interlock/water protection LED that illuminates? Not all the laserscript PSUs have this.