Problem with number of authors and the et al. abbreviation

Hi,
I use biblatex for citations. Unfortunately, the authors are inconsistently truncated with "et al.": Sometimes "et al." appears after one author (which is what I want), sometimes after two authors:
Could you help me to fix the problem? Here is my YAML:
#####################
##  output format  ##
#####################
# The lines below makes the 'knit' button build the entire thesis 
# Edit the line 'thesis_formats <- "pdf"' to the formats you want
# The format options are: 'pdf', 'bs4', 'gitbook', 'word'
# E.g. you can build both pdf and html with 'thesis_formats <- c("pdf", "bs4")'
knit: (function(input, ...) {
    thesis_formats <- "pdf";

    source("scripts_and_filters/knit-functions.R");
    knit_thesis(input, thesis_formats, ...)
  })

#####################
## thesis metadata ##
#####################
title: |
  Forschungskooperationen – \
  Probleme und Erfolgsdeterminanten
author:
college:  |
  Der Philosophischen Fakultät \
  der Gottfried Wilhelm Leibniz Universität Hannover \
  zur Erlangung des Grades eines \
  Doktors der Philosophie (Dr. phil.) \
  vorgelegte Dissertation \
  \

university:   |
  von Malte Hückstädt \
  geboren am 04.01.1983 \
  in Schwedt/Oder \
university-logo: templates/luh.pdf
university-logo-width: 8cm
submitted-text: 
degree:
degreedate: 2022
dedication: Für meinen Opa Karl
acknowledgements: |
  `r paste(readLines("front-and-back-matter/_acknowledgements.Rmd"), collapse = '\n  ')`
show-acknowledgements-in-toc: false
abstract-heading: "Zusammenfassung"
abstract: |
  `r paste(readLines("front-and-back-matter/_abstract.Rmd"), collapse = "\n  ")`
show-abstract-in-toc: false
# add a second abstract with abstract-second-heading: "Abstract two", and abstract-second: "My abstract"
abbreviations: |
  `r paste(readLines("front-and-back-matter/_abbreviations.Rmd"), collapse = '\n  ')`

#######################
## bibliography path ##
#######################
bibliography: [bibliography/references.bib]

########################
## PDF layout options ###
#########################
### submitting a master's thesis ###
# set masters-submission: true for an alternative, anonymous title page with 
# candidate number and word count
masters-submission: false
candidate-number: 123456
word-count: "10,052"

# if you want to use a different title page altogether, provide a path to a 
# .tex file here and it will override the default Oxford one
# alternative-title-page: templates/alt-title-page-example.tex

### abbreviations ###
abbreviations-width: 3.2cm
abbreviations-heading: Abkürzungsverzeichnis

### citation and bibliography style ###
# heading title for the references section
params:
  insertReferencesHeadingInPDF: true
  insertReferencesHeadingInOtherOutput: true
  referenceHeading: "Literaturverzeichnis"

# citation and reference options (pandoc) #
#csl: bibliography/oxford-university-press-humsoc.csl # try csl: bibliography/transactions-on-computer-human-interaction.csl for numerical citations
refs-line-spacing: 6mm
refs-space-between-entries: 1mm

## biblatex options ##
use-biblatex: true # set to true, and set "output:bookdown::pdf_book: citation_package: biblatex"
bib-latex-options: "style=authoryear, sorting=nyt, backend=biber, maxcitenames=2, useprefix, doi=true, isbn=false, uniquename=false, language=auto" #for science, you might want style=numeric-comp, sorting=none for numerical in-text citation with references in order of appearance

## natbib options ##
#use-natbib: true # set to true, and set "output:bookdown::pdf_book: citation_package: natbib"
#natbib-citation-style: authoryear #for science, you might want numbers,square
#natbib-bibliography-style: templates/ACM-Reference-Format.bst #e.g. "plainnat", unsrtnat, or path to a .bst file

### correction highlighting ###
corrections: true

### link highlighting ###
colored-not-bordered-links: true # true = highlight links by coloring text; false = highlight links with a border

# Set the link text/border coloring here, in RGB. 
# Comment out a variable to use whatever the text's existing color is.
# If printing a physical version of your thesis, you'll want to comment out all of these.
urlcolor-rgb: "0,0,139"     # web addresses
citecolor-rgb: "0,33,71"    # citations
linkcolor-rgb: "0,0,139"    # links to sections in your thesis

# Make page number, not text, be link in TOC, LOF, and LOT. Otherwise, section link
# highlighting may look a bit excessive.
toc-link-page-numbers: true

### binding / margins ###
page-layout: twoside
#'nobind' for equal margins (PDF output), 'twoside' for two-sided binding (mirror margins and blank pages), leave blank for one-sided binding (left margin > right margin)

### position of page numbers ###
#C = center, R = right, L = left. If page layout is 'twoside', O = odd pages and E = even pages. E.g. RO,LE puts the page number to the right on odd pages and left on even pages
ordinary-page-number-foot-or-head: foot #'foot' puts page number in footer, 'head' in header
ordinary-page-number-position: C
chapter-page-number-foot-or-head: foot #you may want it to be different on the chapter pages
chapter-page-number-position: C

### position of running header ###
#C = center, R = right, L = left. If page layout is 'twoside', O = odd pages and E = even pages. E.g. RO,LE puts the page number to the right on odd pages and left on even pages
running-header: true #indicate current chapter/section in header?
running-header-foot-or-head: head
running-header-position-leftmark: LO #marks the chapter. If layout is 'nobind', only this is used.
running-header-position-rightmark: RE  #marks the section.

### draft mark ###
draft-mark: false # add a DRAFT mark?
draft-mark-foot-or-head: foot ##'foot' = in footer, 'head' = in header
draft-mark-position: C

### section numbering ###
section-numbering-depth: 5 # to which depth should headings be numbered?

### tables of content ###
toc-depth: 5 # to which depth should headings be included in table of contents?
lof: true # include list of figures in front matter?
lot: true # include list of tables in front matter?
mini-toc: true  # include mini-table of contents at start of each chapter? (this just prepares it; you must also add \minitoc after the chapter titles)
mini-lot: false  # include mini-list of tables by start of each chapter?
mini-lof: false  # include mini-list of figures by start of each chapter?

### code block spacing ###
space-before-code-block: 10pt
space-after-code-block: 8pt

### linespacing ###
linespacing: 22pt plus2pt # 22pt is official for submission & library copies
frontmatter-linespacing: 17pt plus1pt minus1pt #spacing in roman-numbered pages (acknowledgments, table of contents, etc.)

### title page
title-size: 22pt
title-size-linespacing: 28pt
gap-before-crest: 25mm
gap-after-crest: 25mm

### other stuff ###
abstractseparate: false  # include front page w/ abstract for examination schools?
includeline-num: false # show line numbering in PDF?
line-wrapping-in-code: true #wrap overflowing lines in code blocks?

#####################
## output details  ##
#####################
output:
  bookdown::pdf_book:
    citation_package: biblatex
    template: templates/template.tex
    latex_engine: xelatex
    keep_tex: true
    pandoc_args: "--lua-filter=scripts_and_filters/colour_and_highlight.lua"
  bookdown::bs4_book: 
    css: 
      - templates/bs4_style.css
      - templates/corrections.css # remove to stop highlighting corrections
    theme:
      primary: "#6D1919"
    repo: https://github.com/ulyngs/oxforddown
    pandoc_args: "--lua-filter=scripts_and_filters/colour_and_highlight.lua"
  bookdown::gitbook:
    css: templates/style.css
    config:
      sharing:
        facebook: false
        twitter: false
        all: false
  bookdown::word_document2:
    toc: true   
link-citations: true
documentclass: book
always_allow_html: true #this allows html stuff in word (.docx) output
ulyngs / oxforddown

Problem with number of authors and the et al. abbreviation #69
