Set Value Dialogbox not showing on WebClient

Set Value Dialogbox not showing on WebClient

Hello!
On one of our customers client PCs is not possible to open the Set Value Dialogbox to change the value of a variable. On mouse over I get shown that I should be able to select the element but the dialogbox doesn't pop up.
If I change the Numerical Value Element to "Set without Dialog" or "via Element" setting values is possible.
Everything works normal on the server and the other client PCs (of which one is a identical PC with the same configuration)
I tried to delete the ZenWebClient files, re installed the ZenOn WebClient software, IE compatibility mode and copied the zenon6.ini of a working PC but there is still the same issue.
Maybe someone experienced the same issue?
ZenOn 6.21 SP0 Runtime + WebServer
Client PC:
Windows 7 SP1
Internet Explorer 9
ZenOn WebClient 6.51

This is a migrated post! Originally posted on 26.04.2015 by user kicker. Please be aware that information can be outdated.