Sometimes you can "break" a welded knot by rubbing it in your fingers, working the strings toward each other and bending the knot back and forth. You can also carefully slide a needle beneath one of the strands in the knot. Make sure not to retention the string while you are working the knot out.
Any knot I haven't been able to get out with my fingers I've gotten out with my teeth, just bite the strand you can't grip well and then pull the other strand with your fingers lol.
Get a pair of sharp nose pliers then crush the knot in the flat part of the jaws, where there are no grooves. Crush it flat then turn it sideways and crush it flat again. That should loosen up the knot so that you can use a toothpick or something similar to slide into the knot to open it up.
I hold one loop of the knot down with one fingernail, then with the other hand, pull the other loop out of the first one. The key is to pry the string out of the loop, and it should loosen enough to pull apart after that. So hard to describe this method in text, but I’ve gotten all my knots out with only my hands using this strat, so I still think it’s worth mentioning. Does require medium length fingernails.