Hi all
First of all, a big Thanks to Carl who helped me fixing this problem.
So as a summary for all, the problem was caused by a datatype (an extension to tabular), I manually added to "datatypes_conf.xml"
Removing the datatype fixed the problem. I couldn't identify a syntax problem, neither in "datatypes_conf.xml" nor in "~/lib/galaxy/datatypes/registry.py" and "~/lib/galaxy/datatypes/tabular.py". However, renaming it (in all three files) fixed it as well.
Regards, Hans-Rudolf