Debug Assertion Failed … _BLOCK_TYPE_IS_VALID(pHead->nBlockUse)

I have got a really bad memory leak I am trying to fix, but somehow i am not able to delete Objects without triggering this assertation.

I have searched for a solution via Google and have read the Questions on stackoverflow about this Error but I was still not able to find the answer!

Possible reasons to get this Error according to my research:
1. deleting objects more then one
2. shadow copying
3. creating and deleting Objects that are loaded from an external dll
4. creating objects without storing the pointer

BUT:
1. I checked the code and was not able to find double deletion
2. I use a copy constructor to copy Objects
3. The Error relatet classes are build (with MS Visual Studio) to a seperate lib but not to a dll. AND all the classes that are related to this error are located in the same lib.
4. I checked the code and it seems like that's not the problem

It would be great if anybody is able to spot the mistake in the code below, and I appreciate every hint that points me to the solution of the problem.

EDIT:
I forgot to mention the same deleting problem in sendThreadMain of MessageSystem (see code below). If i delete the Message there it causes unexpected errors somewhere else in the code. Might just be wrong data transmission... but i do not really know.
This code is run on Windows and Linux!

Here are the error related parts of the code:

Message

class Message 
{
public:
    Message (char type, unsigned char id, unsigned short size) 
    {
        mType = type;
        mId = id;
        mSize= size;
    }

    Message(const Message &o)
    {
        mType = o.mType;
        mId = o.mId;
        mSize = o.mSize;
    }

    char getType() const {return mType;};
    unsigned char getId() const {return mId;};
    unsigned short getSize() const {return mSize;};

protected:
    char mType;
    unsigned char mId;
    unsigned short mSize;
};


class Jo