After scratching my head - I decided to pull the tombstones out and inspect the wiring to make sure there was no sneaky connections and also nothing out of the ordinary. The older ballasts (original diagram are Magnetic Ballasts with PCB) the new ballast are electronic. I contacted the manufacturer and they suggested to verify my grounds for the ballast assembly. Since I had several that were fine and all inline - I verified ground from good unit to ground on unit that I was shocked with. Grounds were good.
So upon pulling the tombstones out and checking the wiring - I tugged on each of the wires, one of the wires just slid out of the connection. While it was inserted in the tombstone connection just fine; it just kind of slid out as if the spring clamp had not locked it in. I removed the wire and installed a new wire into the tombstone making sure it clamped down on the wire.
After I had done this there was no longer an issue. I assume the wire was close enough to conduct but far enough away to cause the issue.
I do not understand that engineering wise - but maybe it has something to do with RF, High Voltage and an Air gap.
1. With HPS, no for smaller bulbs on bigger ballasts.
Discharge lights are current devices. They are non-linear (behave almost like a dead short) and so, must be externally current-limited. That's the whole point of a ballast or driver.
HPS bulbs work at a specific current in one of four working voltage ranges (page 9): 52V, 55V, 100V or 250V plus or minus 15%. To swap, the bulbs would need to be in the same voltage range. Too small a bulb, the bigger ballast would force the higher current through the bulb, burning it out. The reverse, the bulb might work, but might not warm up properly.
By comparison, if you stick a VHO 8' fluorescent tube in an HO fixture, you get HO light output from the VHO bulb.
2. Nope! Different and incompatible, especially ignitors
You absolutely cannot interchange various types of discharge lighting - not least because the way the ignitors work is very different.
Here, HPS is an odd duck. It has an ignitor which fires a 2000-3000 volt spike to ignite the bulb. The supply is constant-current, so it will increase voltage under no load. This triggers a VBO, which flows a burst of current into a winding on the ballast transformer, inducing a spike to the bulb.
They make special MH bulbs made to run on HPS fixtures; these are resistant to the spike and are tuned for HPS voltage and current. They cost 50-100% more than a stock HPS or MH bulb.
Other than that, facility managers don't intentionally use the wrong bulb. Nobody will support that, so if you want to try it, you're on your own. It will either not work, give poor life, burn out the ballast, or blow bulbs - and these suckers run very hot and you don't want molten sodium raining on your head. Some are even reputed for starting fires.
3. LED replacements must be designed for that.
You linked a compromise screw-in LED meant to go into an HPS or other HID light without removing the ballast. If they are designed, (UL) listed, and labeled for that use, it's safe and legal. Otherwise, it is not. The operative word is "compromise".
HPS is more lumen-efficient than any but the most efficient LEDs on the market - if you like yellow light. And if you're trying to light the inside of a sphere, which you might if you're growing pot. Reflectors suck. If you really want a cone of light (and most people do), LEDs win big, because they inherently emit a cone of light. They are far more lumen-efficient inside that cone, and weak reflectors don't help enough.
Of course, if arranged like a "corn cob", the advantage is utterly lost. SMH... Those are for people in a hurry looking for a screw-in replacement that uses their same reflectors and lensing. But wait. Why keep the old fixtures? Do you really want to keep the old ballast? No, it wastes energy and is a maintenance item. You want to direct-wire existing line voltage to the LED device. Do you honestly foresee rolling back to actual HID lighting? Never gonna happen. Throw the fixture in the attic, get a modern LED fixture for 1/3 the cost - and don't look back.
Standard practice is replacing a 400W HID with a 6-tube F32T8 fixture. 4' LED replacement tubes are a commodity item, and prices are in free-fall -- $8 last I checked for a top brand. $48 and you have a HID light's worth of LED light. Hit up Craigslist for a used fluorescent fixture, yank the ballast and rewire direct - you're in business. Given how cheap LEDs have gotten I wonder why anyone would fool around with HID lighting anymore.
Real fluorescents are even cheaper if you get eBay ballasts, and the superb tubes now available.
Best Answer
The given phototherapy tube is a glow start tube with filaments. The glow starter is built into the base. Glow start tubes are usually driven with conventional magnetic ballasts. There might be also electronic ballasts for 2 pin tubes with built in glow starters, but usually electronic ballasts need access to all 4 pins of the tube.
The given listing for "universal electronic ballast" is lacking in details (input voltage/frequency ranges, tube current etc.) to give any recommendation for it's suitability.
I'd recommend replacing the existing ballast with an European model. For example: http://www.lighting.philips.com/main/prof/lighting-electronics/fluorescent/fluorescent-electromagnetic/bpl-em-ballasts-for-cfl-pl-t-pl-s-and-pl-c-lamps/913710122350_EU/product or https://www.helvar.com/en/products/L11D240V50Hz/ or any other magnetic ballast with correct frequency, voltage and lamp power. Also when converting the wires might need also upgrading for the higher mains voltage. Consult a local lamp repair shop for advice.
Added, since unable to comment:
Suitable search terms to try on your favorite online platform: "magnetic ballast 9w" "Helvar L11D" "pl-s ballast".