Excel Exits Macro Without Warning - No Errors

I'm writing a piece of code that puts certain values in certain cells based on certain criteria. (Vague, I know.)

When the code gets to the part where it puts the first value in the first cell (in a loop), it inserts the value and then the code stops running. Even when I put in "On Error Goto Err" with a msgbox that contains the error description, the code just quietly stops.

Here is a simple macro I wrote to see if maybe the problem had anything to do with something specific to my code. This one fails too, for no apparent reason. Could my file be corrupted or something? What do I do about this?

Code:
Sub rtest()

On Error GoTo err

Sheet2.Range("p4") = "test" 'code stops on first line, whether it is this one or the next every time.
ActiveCell.Value = "test"
ActiveCell.Value = "success"

End
err:
MsgBox err.Description

End Sub