Not sure about the error, but the tab and F keys should be Sess0.Screen.SendKeys "<Tab><Pf1><Pf13>"
You should not need to send the TAB key. Use the PutString method to put data straight to a field.
Sess0.Screen.PutString "MyString", X, Y
Then you can use:
Sess0.Screen.SendKeys "<Enter>"
or whatever other action key you need. We use F1 alot for action:
Sess0.Screen.SendKeys "<Pf1>"
As to the error, sometimes the GetString function performs strangely when you get long strings. You are getting a large string, are you just storing it or are you comparing it to something? Just a thought.
calculus