I am running on ARServer 5.0, midtier 5.0.1 on a Windows platform with SQL Server,
Typically I have noticed that when you have such a problem, redeploy the application, or update the cache.
But since the patches I am not having these problems.
YEs, Open Window does work on mid-tier.
When you redeploy, do a clean redeploy, meaning you need to delete the files from NEw Atlanta folder and for the APPS folder in mid-tier.