Firstly I'm very sorry because this question was in fact answered already. I only have a question to the following answer and I'm not allowed to comment there directly: https://tex.stackexchange.com/a/24197/73704
I liked this response and Leo's small example worked perfectly. Unfortunately, the document I use is quite big and has already a parskip defined (it should stay in there).
I adjusted his example to fit my scenario and the problem becomes visible: there is a vertical space after "foo".
\documentclass{article}
\parskip1.5ex plus0.5ex minus0.5ex
\newenvironment{tightcenter}{%
\setlength\topsep{0pt}
\setlength\parskip{0pt}
\begin{center}
}{%
\end{center}
}
\begin{document}
text text text text text text text text text text text text text text text text text text text text text text text text
\begin{tightcenter}
foo
\end{tightcenter}
text text text text text text text text text text text text text text text text text text text text text text text text
\end{document}
Does anybody know why this problem appears and how it can be solved?