That mangled up washer did not come from your AR. You've been given some good trouble-shooting advice here.
First, let me state my understanding of the problem so we can be sure I know what we're all looking at. This is a rifle that functioned just fine for the first magazine, and now without explanation will function as a single-shot only. Upon pulling the trigger, the gun discharges but the bolt does not cycle. Upon racking the charging handle an empty is extracted and ejected, a new round is fed to the chamber, and the rifle can be fired again. All is correct?
If so, follow the advise you have been given about running some WD-40 through the gas system. Stick the hose into the receiver end of the gas tube, and blow WD out through the barrel. Then, blow WD in through the end of the gas key to ensure it runs out through the middle of the bolt carrier. Somewhere, something is blocking the gas system. My guesses are foreign debris in the gas key (I know you already checked visually, but maybe something made it past the corner), or your gas block has shifted and closed off the gas port. Looks to me like your gas block would be fastened with set screws instead of taper pins. Check to see if any of the screws are loose, or if the gas block has shifted toward the muzzle end of the barrel. There is a shoulder on the barrel that the gas block should be pushed down against. Is there a gap between the gas block and this shoulder?