Thanks for you help.
I finally got it to work and pinpointed the problem. I if add my new library to the already existing group/module (which was created by the project template), it works. But if I try to create an additional group and put the new library there, it does not work.
Your step-by-step instruction does not actually specify which group the new library should be added to; it just says you can change it. It is not obvious which one you should use. When you choose a new library the name of a group (module name) is automatically generated and thus I thought that name should be used. I don't know if the problem I encountered should actually be a problem, but if it is, it could maybe be clarified in the GUI that the same group as the existing one must be used? However, I don't see the point in this kind of restriction so I hope it is just a bug.
Also the manual is not clear about the meaning of groups of libraries. You mention that in autotools libraries are grouped, but why? Could the creation of groups be optional?