Constants, fixed-length strings, arrays, user-defined types and Declare statements not allowed as Public members of object modules

Declaring the following in a Userform is one possible cause for the error:

Declare Function apiFindWindow Lib "User32" Alias "FindWindowA" _

(ByVal lpclassname As Any, ByVal lpCaption As Any) As Long

DLL procedures declared in standard modules are public by default and can
be called from anywhere in your application. DLL procedures declared in any
other type of module are private to that module, and you must identify them
as such by preceding the declaration with the Private keyword. Hence

Private Declare Function apiFindWindow Lib "User32" Alias "FindWindowA" _

(ByVal lpclassname As Any, ByVal lpCaption As Any) As Long

Should solve the problem