You have upgraded your older GW client to a version 18 and enabled a conversation threading feature (Tools | Options | Environment). When you want to reply on a mail that has few threads already, you will see in those threads a security warning that GroupWise was not able to load a page.
This does not happen on workstations which did not have older GW client installed before and you just installed newly GW18 client.
Run install process via standard Add/Remove tool of Windows. Delete any leftover directories of a GW client where it was installed before. Check if there are any GW related files in c:\Windows\SysWOW64 .. or perhaps also in C:\Windows\System32. Clean Windows registry settings -> HK_CU->Software->Novell, remove GroupWise directory.
Install newly GW18 client.