Strange output with a LM7171 opamp (non-inverser amplifier)
Several observations

1) The data sheet shows 511ohm resistors being used as feedback, you've got 5.11kohms. This is a very fast amplifier. The low value resistor they suggest is to control poles caused by input capacitance. Use the values they suggest.

2) The data sheet suggests a feedback capacitor of a few pFs across R2 to further control input terminal poles. If you are only used to implementing low speed opamps, this wouldn't have bitten you before.

3) Are you driving your scope via a cable to the input? These amplifiers don't like driving capacitance directly. A metre of cable and a scope input could easily be 150pF. The data sheet suggests a 50ohm series resistor before any capacitor. As you want a 50ohm output anyway, this would not be a problem to you.

Any one of these could cause oscillation. All three issues together? Problem guaranteed.