Hi,
I have an alternative theory, based on personal experience with my Edellux front light and SON 28. My light went out during a very wet bike trip in Norway. Back home, I opened the light and found much moisture inside, partly covering the electronics. Further forensics showed the the elastomer O-ring, normally sealing the front glass to the housing, had become brittle and broken at several locations. (I suspect that the O-ring material was chemically incompatible with cosmetics carried out by previous owner. This could include applying a polishing wax or even worse, spraying frame with a chemical). I dried the light internals and installed a new O-ring. My Edellux has worked flawlessly since. More details can be found elsewhere on this forum.

To elaborate my theory;  it would not come as a surprise if the moisture in the light also affected the current supplied to the "Sinewave". Perhaps the SON 28 was perfectly innocent after all.

Just a thought……the OP had a problem with gear slippage in the lower ratios and then changed the oil (and adjusted the chain) which sorted the problem……then found the oil leak.

Could an oil leak have been there all the time (and emptied the hub of oil) which has now become more obvious with new (more) oil in the hub?

Did much oil come out when the hub was drained or was it dry?

When I put my EX box together and do not get all gears, say if the shifter only goes from 1 to 12, I put the hub into that gear the shifter says is gear 12.  But I know that I actually put it in the highest gear that way, so I take the cover off the EX Bos, then turn the shifter to 14 and reassemble.

Or if the shifter only worked from gears 4 to 14, same concept.  Set it at 4 which is the lowest gear, take the cover off the EX Box, set to gear 1, reassemble.

Takes less than two minutes, no tools needed.

Thank you

It sounds from what eveyone has said that it's probably not worth doing a repeat oil change 7 days after the previous change to cure this issue (apart from peace of mind maybe)?

If I'm looking at the inside surface of ex-box face on, do I turn the ex-box nut clockwise or anti-clockwise for gear 14?

Thanks for that - reassuring!

After further inspection this morning, I found v similar black watery fluid underneath the Ex-box so suspect the "leak" was coming from here (not the port on the main hub

However whilst investigating this I stupidly put the shifter in gear 1 (not 14) before detaching the ex-box and cleaning it a bit

Now it won't reattach properly - how do I realign things to gear 14 so I can attach it back correctly?

Can I push the ex box on as far as possible then use shifter to move the ex-box to gear 14? Or do I need to use a spanner on the ex-box?