Thank you everybody for your answers.  Writing Tr{\"a}ger as Thomas suggested works well, but unfortunately, I'm using Mendeley Desktop for the management of my bibtex file and I can't seem to be able to influence the way in which it encodes the special characters.

@Mojca: Indeed, it also fails if I just write "Träger" in UTF-8 encoding (however, "Schräger" works just fine. All combinations where the "ä" is at the third place of the word seem to fail.). The error message is similar but slightly different now. The log file shows the following:

---------------

system > begin file test.tex at line 3
publications > loading database from test.bbl
(test.bbl
! String contains an invalid utf-8 sequence.
l.1 \setuppublicationlist[samplesize={Tr
Ã06},totalnumber=1]
A funny symbol that I can't read has just been (re)read.
Just continue, I'll change it to 0xFFFD.

! String contains an invalid utf-8 sequence.
l.5 n=1,s=Tr
Ã06]
A funny symbol that I can't read has just been (re)read.
Just continue, I'll change it to 0xFFFD.

! String contains an invalid utf-8 sequence.
\doifassignmentelse ...gnmentelse \detokenize {#1}
=@@\@end@ \expandafter \se...
\dostartpublication ... ->\doifassignmentelse {#1}
{\getparameters [\??pb ][k...
l.9 \stoppublication

A funny symbol that I can't read has just been (re)read.
Just continue, I'll change it to 0xFFFD.

)

-----------------------

Julian