Regarding the hint with the different threads: I wrote a UnitTest which calls the method from two different threads and both work fine. So at least the different threads are not a general problem.
Regarding the hint with opening and saving the Report Definition file in the C1Report Designer: Is there a known reason why this should work?
The problem is, that I cannot reproduce this problem and that the client is quite far away and I do not have the possibility to access the machine where the problem occurs. Therefore trial and error is difficult.
Any more hints are welcome.