Got an error message suggesting there there is an INSTALL file which is missing the name of the icon:

file INSTALL cannot find
"/home/travis/build/AppImage/appimaged/resources/appimaged.png".

But no file named INSTALL exists.

Misleading wording.

An experienced CMake user pointed out to me

Well, to me, it's pretty clear, there's different install(...) commands, one is install(FILE ...), which it refers to.

Would have cost me a lot of time searching the problem in the entirely wrong places.

Reference: https://github.com/AppImage/appimaged/pull/99#issuecomment-552189475