I believe your problem is similar to mine. I don't think this is to do with your Fritzbox (I get the same results on my Zyxel and Draytek, in addition to my Fritzbox).
In my case, there is a high resistant fault (something to do with jelly crimping) that openreach won't fix because of where the cables are and how accessible they are. I believe there's also noise on the line (it seems that when our neighbours are in, and they are not in often nowadays, and when they put something on, presumably the boiler or water tank heater, for every 2 errors on the line (ES on fritzbox), I usually get one SES, but this, I believe depends on how intensive the noise thing is, which makes me think it's a hot water tank). During this the min effective data rate also gets reduced to 3kbps).
So far, I've been trying to resolve our issues for over a year now since we started working from home and switched over to VDSL. Lots of testing, lots of money spent, too, lots of emails to Plusnet and lots of openreach visits (14 at least). Openreach weren't transparent with us: two mentioned a high resistance fault, whereas another admitted they didn't know where the interference was coming from, but because no-one complains from the neighbours who also have the problem as confirmed by them and BT wholesale (all retired and don't use the internet as much), they'll not investigate just one flat.