\documentclass[12pt,halfparskip]{scrartcl}
\usepackage[style=authoryear, citestyle=authoryear, sortcites=true,sorting=nyt,backend=biber,pagetracker=true,ibidtracker=constrict, maxcitenames=2]{biblatex}
\DefineBibliographyStrings{german}{urlseen = {zuletzt abgerufen am}, }
\DeclareDelimFormat{finalnamedelim}{\addspace\&\space}
\begin{document}
\footcite[53\psq]{Klett2015}
\end{document}
.bib file
@Book{Klett2015,
title = {Lambacher Schweizer -- Mathematik Qualifikationsphase Grundkurs},
publisher = {Ernst Klett Verlag},
year = {2015},
author = {Baum, Manfred AND Bellsted, Martin AND Brandt, Dr. Dieter AND Buck, Heidi AND Dürr, Prof. Rolf AND Freudigmann, Prof. Hans AND Greulich, Dieter AND Haug, Dr. Frieder AND Jörgens, Thomas AND Jürgensen-Engl, Thorsten AND Riemer, Dr. Wolfgang AND Sandmann, Rüdiger AND Schmitt-Hartmann, Reinhard AND Sonntag, Raphaela AND Spielmans, Heike AND Wollmann, Alexander AND Zimmermann, Dr. Peter AND Zinser, Prof. Manfred},
address = {Stuttgart}}
I've tried to make a MWE (I don't how to input only one reference without an extra file). Unfortunately only the book 'Klett2015' doesn't work with \footcite. I get this:

maxcitenames doesn't seem to be considered. Any ideas?
Appreciate your help!
maxcitenameson my system. I get the following output: https://i.stack.imgur.com/4SYFd.png – leandriis Jan 03 '20 at 10:35uniquelist=false, see https://tex.stackexchange.com/q/69028/35864. Note that usually academic titles and degrees (Dr., Prof.) are not given in theauthorfield.biblatexdoesn't deal with them particularly well, either. – moewe Jan 03 '20 at 10:36style=authoryear, citestyle=authoryear,is redundant and equivalent to the simplerstyle=authoryear,. With that style settingsorting=nyt,is also automatically set, so need not be given explicitly.style=authoryear,also setspagetracker=true, so it too need not be set explicitly. Setting a value foribidtrackeris not really useful as the tracker will not be used byauthoryear(it would be used withstyle=authoryear-ibid,, whereibidtracker=constrict,is the default). – moewe Jan 03 '20 at 10:40.bibfile. (Plus the changes I suggested above.) – moewe Jan 03 '20 at 10:45uniquelist? – jacmeird Jan 03 '20 at 11:23maxbibnames=999,(or some other ridiculously large - or sensibly large - number, see https://tex.stackexchange.com/q/1554/35864). – moewe Jan 03 '20 at 11:33uniquelist, so that it is a duplicate of https://tex.stackexchange.com/q/69028/35864? – moewe Jan 07 '20 at 22:16