I have just had this issue ( OS: 5.5.2.13463 ) and thanks to the above posters my fix was also found in the Output section
Aux to headphones was on 0.8.
As soon as I rolled it back to 0, it started to work again 
FX LOOP Not working after Beta Update
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grateful -
May 12, 2018 at 7:53 PM -
Thread is marked as Resolved.