I'm no C++ expert and without having your code it's really hard to say what's wrong.
I bet u have already googled a lot on this issue, but could this help?

Especially this part:
"Circular Dependencies vs bidirectional relationships

Whenever one file #includes another, there is a strong dependency created between the files. When a dependency like this exists between header files, it can not be bidirectional: the preprocessor is unable to cope with a circular dependency between qheader files, where each one #includes the other. One of the #include statements must be replaced by a forward class declaration.

Forward declarations help remove circular dependencies between classes, and in the process, enable bidirectional relationships to exist between them."

Are u by any chance including the Delegate twice in two different classes?