Sticky residues don't bode well for good connections. My first thoughts are that it's the result of melting insulation from overheated cables, but that would probably re-set hard again when cool?
You would probably be better off cutting back to clean cable, than trying to clean what you have (contaminant unknown), provided there is sufficient length.
I use 4mm soldered Gold bullets, which I think are good for 50amps. RC model shops are a good source, or like everything else, just order online.
GWS is one such online source, but they never let go once they get your email address, so be warned.
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