That looks like epoxy. I don't think anything commercially available will dissolve it. It may be difficult to remove that end cap without wrecking the bar. Just to start, I'd clamp the end cap in a vise, and gently twist the bar back and forth. The cap may break loose, but you'll probably still have to get the epoxy off the bar. You'd likely have to grind it off.

Yeah, good ol' MEK. Back in the '70s, professional bowlers used to soak their polyester bowling balls in it to soften the shell. This would allow them to more easily impart a big hook on the bowling ball in competition. I didn't know it would dissolve epoxy though.