Ilfosol S is a one shot developer and had a reasonable shelf life as a concentrate, but had a reputation for collapsing if kept too long once opened and becoming useless, it was discontinued some time ago, (around 2005). This year it was replaced by Ilfosol 3. However it sounds like your Ilfosol S is still OK.
First it should only be diluted just before use, and you only make up the quantity you need for each film in the dev tank. I'm puzzled at your dilution "ILFOSOL S is available in North America in 500ml bottles" so your 1+9 dilution should be 50ml + 450mm to make 500ml, thta's a typical volume for a 120 film, or 30mm +270ml to make 300ml the typical volume for a 35mm film.
So it sounds like you've added 500ml to 3.785 litre = 1 US gallon. Either way that solution now has a very short lifespan
"WORKING SOLUTION LIF . . . . and ILFOSOL S working strength solutions should not be kept for more than 24 hours. Make up fresh developer each time it is needed and discard it after the processing session."
We all had these problems when we statrted, you have to learn by mistakes
You should go and look at the Ilford website they have an excellent section on
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Ian