Moreover, I noticed the the library project does not produce the file "mylib.a" (MinGW target, the file name is specified as the output file), but "libmylib.a" instead:
Linking static library: ..\..\Libs\mylib.a
ar.exe: creating ..\..\Libs\libmylib.a
ranlib: '..\..\Libs\mylib.a': No such file