Serienbrief aus CSV mit datatool läuft in "File ended while scanning use of \\@DTLforeach" ohne PDF zu erstellen

Ich habe dann einfach weiter geforscht und nachdem ich weder mit \ifx, ifthenelse oder einfach nur if weiter kam, stieß ich bei Stackexchange auf den wesentlichen Punkt: Der Adressbereich von \begin{letter} verträgt sich nicht mit u.a. \ifthenelse.¹ Auch wenn mir die Information an sich noch nicht weiterhilft und ich keine Ahnung habe warum die bisher nachweislich mehr als 100 Briefe kein Problem waren, war dort immerhin ein Workaround dabei; nicht schön, aber funktioniert.

Hier die Lösung (neues \makeaddress anstatt \begin{letter}) bei mir angewendet:

%example data
\begin{filecontents*}%
%~ [overwrite] % good to know it exists, but not needed here
{data.csv} Memberno;Mark;Nick;EMail;PGP;Firm;Title;FirstNames;FamilyNames;Phone;PostalNames;AddressDetails;Street;HouseNumber;ZIP;Place;Country
1;m;Sandy;sandy@strahlenau.de;;;;Sandy;Sonnenschein;;Sandra Sunshine;;Wetterstraße;5;11111;Strahlenau;
2;m;York;york@gmail.com;0x12345678;NoGo GbR;Dr.;York;Quietsch;;;;Bachstraße;4;67345;Ententeich;Österreich
3;p;Sab;sab@gmx.net;0xabcdef;;;Sabine;Müller;;;;Kutscherstraße;19;39114;Sonstwo;
4;m;Jane;jane@1und1.de;;;;Hannah;Geisler;;;;Hauptstraße;44 a;1789;Sity;Schweiz
\end{filecontents*}

\documentclass[
  fontsize=11pt,
  paper=a4,
  parskip=full,
  enlargefirstpage=on,    % More space on first page
  fromalign=left,        % PLacement of name in letter head
  fromphone=false,
  fromfax=false,
  fromrule=aftername,     % Rule after sender name in letter head
  fromemail=on,           % turn on email of sender
  fromurl=on,
  addrfield=on,           % Adress field for envelope with window
  backaddress=on,         % Sender address in this window
  %~ subject=beforeopening,  % Placement of subject
  locfield=narrow,        % Additional field for sender
  foldmarks=on,           % Print foldmarks
  version=last,
  pagenumber=off,
  subject=untitled,
  ngerman,          % modern german orthography
]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{scrletter}
\nonfrenchspacing

\setkomavar{fromzipcode}{12345}
\setkomavar{place}{Place}
\setkomavar{date}{%
\today%
}
\setkomavar{fromemail}[]{mail@sender.com}
\setkomavar{fromurl}[]{https://domain.sender.com/}

\LoadLetterOption{DIN}

\setkomavar{fromname}{SENDER Company}
\setkomavar{toname}{RECEIPIENT}

\setkomavar{fromaddress}{%
Streetaddress \\
\usekomavar{fromzipcode}~\usekomavar{place}
}

\setkomavar{backaddressseparator}{\enspace-\enspace}
\setkomavar{enclseparator}[Attachments]{: }

\usepackage[utf8,separator=;]{datatool}
\DTLsetseparator{;}
\DTLloadrawdb[
keys={%
fieldmemberno,fieldmark,fieldnick,fieldemail,fieldpgp,fieldfirm,fieldtitle,fieldname1,fieldname2,fieldphone,fieldnamepostal,fieldremark,fieldstreet,fieldhousenr,fieldzip,fieldplace,fieldcountry %
}
]%
{addresses}%
{data.csv}

\renewcommand*{\raggedsignature}{\raggedright}
\setkomavar{signature}{boardmembers}

% found at https://tex.stackexchange.com/questions/192669/mail-merge-using-datatool-with-letter-class
% The argument to \begin{letter} is massaged in order to extract the addressee. This massage is incompatible with \ifthenelse.
\makeatletter
\newtoks\address@toks
\newcommand\addto@address[1]{%
  \address@toks=\expandafter{\the\expandafter\address@toks#1}%
}
\newcommand\makeaddress{%
  \address@toks={}%
  \ifthenelse{\DTLiseq{\addrFirm}{}}{}{\addto@address{\addrFirm\\}}
  \ifthenelse{\DTLiseq{\addrTitle}{}}{}{\addto@address{\addrTitle\ }}%
  \ifthenelse{\DTLiseq{\addrPostname}{}}{\addto@address{\addrFirstname\ \addrSurname\\}}{\addto@address{\addrPostname\\}}
  \ifthenelse{\DTLiseq{\addrRemark}{}}{}{\addto@address{\addrRemark\\}}
  \ifthenelse{\DTLiseq{\addrStreet}{}}{}{\addto@address{\addrStreet}}
  \ifthenelse{\DTLiseq{\addrHouseNr}{}}{}{\addto@address{\ \addrHouseNr}}
  \addto@address{\noexpand\\}
  \ifthenelse{\DTLiseq{\addrZIP}{}}{}{\addto@address{\addrZIP\ }}
  \ifthenelse{\DTLiseq{\addrPlace}{}}{}{\addto@address{\addrPlace}}
  %~ \ifthenelse{\DTLiseq{\addrState}{}}{}{, \addto@address{\addrState}}
  \ifthenelse{\DTLiseq{\addrCountry}{}}{}{\addto@address{\\\addrCountry}}
  \begingroup
  \edef\x{\endgroup\noexpand\begin{letter}{\the\address@toks}}\x
}
\makeatother

\begin{document}
\DTLforeach*{addresses}{%
    \Chiffre=fieldmemberno,\Mark=fieldmark,\MemberNick=fieldnick,\MemberEMail=fieldemail,\PGP=fieldpgp,%
    \addrFirm=fieldfirm,\addrTitle=fieldtitle,%
    \addrFirstname=fieldname1,\addrSurname=fieldname2,%
    \Phone=fieldphone,%
    \addrRemark=fieldremark,\addrPostname=fieldnamepostal,
    \addrStreet=fieldstreet,\addrHouseNr=fieldhousenr,\addrZIP=fieldzip,\addrPlace=fieldplace,\addrCountry=fieldcountry
}{
\ifstr{\Mark}{m}{
\setkomavar{title}{Your Membership at \usekomavar{fromname}}
\setkomavar{subject}{}
% begin letter
\makeaddress

\opening{Hello \MemberNick{}!}

Letting you know some detail about your mebership at \usekomavar{fromname}\\

Chiffre: \Chiffre{} (please mention when contacting)
Nick: \MemberNick{} 
E-Mail: \MemberEMail{}

\closing{Yours,}

\end{letter}\newpage
}{}
}
\end{document}

Hier nun auch nicht mehr scrlttr2 als Klasse, da jemand meinte das wäre nicht gut bzw. Zukunftssicher.

Falls noch jemand Verbesserungsvorschläge hat, immer gerne :)