Bibtex: I didn't find a database entry for

To answer my own question:

After I replaced:

regime \citep[][]{Jager2008}.

with:

regime \citep[][]{Jager2008}.

everything works without problems.

Note: I can not see any difference, but the new command was copied from a different location in the latex file (I also copy&pasted the new and old version in here...). This behaviour is reproducable, that means if I click 'undo' and then run again, the same warnings appear again...