WebApplication.CallBackResponse.AddJavaScriptToExecute call causes Access Violation

[oliver]

I get an exception on WebApplication.CallBackResponse.AddJavaScriptToExecute (Exception class name = EAccessViolation) when two users try to access the same page at the same moment. 
I have no problem as long as there is just one user displaying the page.

What do I miss ?

Thank you for your support.