As of this morning (UTC, 2018-05-17) this work-around is not needed any more. Both MikTeX and TeX live now ship an updated version of the newtx package that does not load etextools any more. Update the newtx package.
This is a temporary problem with version 1.55 of newtxmath. It loads the etextools package, which is incompatible with biblatex. Version 1.551 released just a few days later removes the dependency on etextools and is compatible with biblatex again. See also this discussion on the tex-live mailing list for the specifics of this issue as well as https://github.com/plk/biblatex/issues/669 and biblatex and autonum don't work together for general comments on etextools and biblatex.
etextools (re)defines the macro \forlistloop in a way that is incompatible with the original definition from etoolbox that biblatex expects. (etextools is an extension of the ideas of etoolbox and if I understand correctly, etextools got in first when it comes to defining \forlistloop. etextools implemented the macro in version 2i dated 2009-08-31, while etoolbox defines its \forlistloop in version 2.0 dated 2010-08-21.) This is an unfortunate clash of names that can lead to some problems, most notably with biblatex. As far as I am aware etextools was always considered more of an experimental package than one for production use (see also package autonum needs the obsolete etex package and in particular this statement by the package author: https://groups.google.com/d/msg/comp.text.tex/VRrFB4ll5n0/1IVvhFyKqsAJ)
In the meantime and only with biblatex 3.10 or 3.11 you can use
\documentclass{article}%
\usepackage[english, german]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{etoolbox}
\let\oldforlistloop\forlistloop
\usepackage{newtxmath}
\let\forlistloop\oldforlistloop
\usepackage[noerroretextools]{biblatex}
\begin{document}
Hallo
\end{document}
as a workaround. The option noerroretextools will probably be removed in future versions of biblatex.
newtxmathagain. The developer has removed the dependency onetextools(this may not be in MikTeX just yet, you may have to wait; but it is definitely in Tex live). – moewe May 16 '18 at 22:35newtx. You'll have another day or two for the update to become available. – moewe May 16 '18 at 22:59