error opening XLS caused by XLA?

Hi all,

I've got an addin that causes the following error:
"EXCEL.exe generated errors and will be closed by Windows"
This happens when 1) the XLA or it's source-XLS is loaded AND 2) another XLS
is being opened.

Sample code:
"Option Explicit
Public WithEvents AppEvents As Excel.Application

Private Sub AppEvents_WorkbookActivate(ByVal wb As Excel.Workbook)
Call Activate_App(wb)
End Sub
"

Dubugging shows (like) everything is OK, until the end sub statement.
Statements executed within Activate_App are two if's leading to an empty
else-block, nothing else.

Does anyone have a solution? A clue what to look for instead is also welcome.

Thanks in advance.