Looks good, like I said above its not "Black" but that's an easy fix.
Efficiency is basically how much you got out of what is a available as a percentage. Extract is all the soluble parts of the malt, to calculate the mass of extract the equation, Mass of Extract = Volume *Specific Gravity *Plato.
Plato is the other way to measure wort density, it is W/W as %. If you dissolved 125g of sugar in enough water to make 1000g you would have a 12.5% Weight of sugar in 1000g (that's 125/1000 or 0.125). there is a relationship between SG and Plato (oP), roughly SG=(4*oP)/1000+1. The V*SG part of the equation gives you the mass of your 21L.
ME=21*1.050*0.12 = 2.646kg of extract.
Your malt will have a potential (what you would get from a lab test) of about 76%, you used 5.5kg so your potential is about 5.5*0.76 = 4.18kg
Your efficiency would be 2.646/4.18*100 = 63.3%.
About the best you will get on a Guten is something like 80% (that's from other peoples reports, I haven't played with one).
So to get more efficiency.
L:G (Water to Grain) at a pre-boil volume of 26L assuming you took out about 5L with the expended malt looks like you strike water volume was around 31L. your L:G would be roughly 5.64:1.
I don't know if you are sparging at all, bring the L:G back to around 5:1 would give you about 5.5L to sparge with, will get some more extract out of the grain bed. Taste your expended malt, if its sweet, that's left behind extract and you need to sparge better.
Look at your crush. To work effectively all recirculating systems need a fairly coarsely crushed grain bill so the liquor (brewing term for all the water that is used in making beer) can penetrate the grain bed, liberate the enzymes and extract the sugars. Depending on the quality of the mill, but as you crack more coarsely you get more uncrushed malt from which you get very little extract.
At the end of the mash, dump out your expended grain and sort through it, uncrushed grains show up like little pearls in sand, more than 1-2 in a big double handful and you need to crack finer.
Too fine (Cracking grain really is a Goldilocks proposition - get it just right) a crush and you wont get enough flow through the grain bed. look at the expended malt, if its blotchey (darker and lighter in patches)shows uneven flow through the grist, look for dough balls (dry malt in a lump)
The problem with all the cheap recirculating systems I have seen is that its very hard to tell how much of the recirculating liquor id going through the grain and how much just overflowing and going down the outside.
Braumeister pumps up through the grain bed so all the liquor overflowing the malt pipe is going through the grain - its a better system but Spidle own the patent. If you can control the recirculation so all of it is going down through the malt rather than overflowing (maybe just occasionally during the mash so you can see the permeability of the grain bed).
Getting all the other basics right, pH, enough Calcium, check the real V readout temperature will all help. Do a mash out, heat to 80oC or close to at the end of the mash.
Do a full hour mash, a 60-90 minute boil... Get the basics right.
Reading what you have posted I doubt its any one thing more a bit of A+B+C... but look at your crush first it looks like a probable.
Mark
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