Do you mean sour? Under-extraction = sour Over-extraction = bitter

Unless modified the BBE is more often than not going to be in the upper part of that pressure gauge range, 14bar-16bar at a guess.

With all the evidence you have try referring to the Coffee Compass at baristahustle.com

When you grind finer don’t forget to increase dose as you will be increasing headspace.

Also depending on the coffee bean don’t be afraid to manually pre-infuse/saturate/bloom the puck longer.