Grounding Issues with this circuit?
The resistors in series with the LEDs in the solid state relays must be connected to the ground of the processor.

Think about it.  The current to drive a LED goes out a processor digital pin, thru the LED, thru the resistor, and has to get back to the processor ground somehow.

The SSRs give you isolation, so there is no need for the 24 V power supply to be connected to the processor power or ground in any way.  While a single connection, like tying the grounds together, is possible, there is no need for that.  To reduce noise from the solenoids getting back to the low voltage circuitry, I would keep the two power regimes isolated.