@Preben Alsholm: Thanks alot for the answer that makes it clearer to me!

Sorry for the very late respons. I couldn't lock in the last days!

Thanks again,

Markus

Sorry, now the link should work!

@Preben Alsholm 

Thanks a lot for the answer! I understood as well that dsolve had to come up with the integration variable to find a solution but I do not really understand why. As far as I see it I speciefied enough boundary values.

I added my worksheet. The crucial part comes at the end

Adm_maple.mw

Thank you very much for the help! I appreciate that a lot!

Best, Markus