Similar result to Jotman, but I know the why. Just don't know exactly how to rectify.

Was in a hurry and forgot to sand one part of the wall surface before painting one coat of vinyl matt over old gloss. Several months later, paint is cracking where I forgot to sand. (alligator effect)

Solutions? Do I have to sand back down to the original gloss coat? Or will sanding the matt coat be enough? Or are chemical strippers better for a goof like this?

Any tips would be great!

Make sure you didn't paint laytex over oil first. If it is just an isolated spot, just try and sand the one area with 150grit and then 220 grit, after that use some sheetrock mudd to patch of the area, maybe 2 coats will do and then lightly sand that, prime it, if you don't your see the spot, then paint it with the roller to coats,

If it was latex over oil, long term it could be a problem. If the places you sanded are fine, if you go back and sand or strip down to the gloss, and sand the surface like you did the rest, you should be fine. It is possible the cracked paint will scrape off.