By "many employees", would you mind to clarify how many records do you have in your gantt chart, that is, what is the size (the number of elements) in the data arrays?
If there are many bars (eg. over 10000 bars) in the gantt chart, there is a possibility of out of memory or resource issues when using hot spots. A hot spot is like a small push button on the screen. It can response to mouse events (mouse click, pop-up tool tip, etc). Plotting 10000 or 100000 bars are easy for ChartDirector. However, creating 10000 push buttons might take up a lot of memory or resources. For large data sets, the image map often consumes much more memory than the chart itself.
I remembered I provided a simple example of a zoomable and scrollable gantt chart in VB6 to you around a month ago. In that example, all data are passed to ChartDirector, even if only a small part of the data are visible due to zooming in. This is for simplicity and it should work for a few thousand bars. If you have a large number of bars, it is suggested you only pass the visible data to ChartDirector to reduce memory consumption of the image map. I have attached an example for your reference.
If you must display a large number of bars in one chart, you may consider using an "If" statement to disable the image map when the number of bars exceed a certain limit.
If the above is not the cause of the problem, is it possible to try to break the image map code into two lines and save the image map like:
Dim temp As String
temp = c.getHTMLImageMap("clickable", "", "message need to show")
Debug.Print "Size = " & Len(temp)
... some code to save the contents of "temp" into a file ....
viewer.ImageMap = temp
With the above code, if the c.getHTMLImageMap works normally (that is, the code crashes in the "viewer.ImageMap = temp" line), the debugger should display the size of the image map. Please also try to add some code to save the image map to a file. If the file is a few MB (after zipping), you may email it to me for analysis. My email is firstname.lastname@example.org. If the image map is very large, you may need to find a place to upload the file so I can download it.