Need a little help with StreamWriter. Writing to a text file....

Hello all.
I have a generic handler (.ashx) written in C# which a web form calls.  The handler will log all activity to a text file on the server and do a bunch of other stuff ...yet to be implemented..and honestly, I'll be glad just to get this simple logging function working.  It seemed to work fine when I was blindly adding lines to the txt file but I wanted to take it one step further and COUNT the number of lines in the log.  Once the file got to be a certain size and/or have a certain number of line numbers, I would take measures to reduce the file size.  Delete lines....delete the file....etc.  But once I added this additional 'CountLogLines' method, it started failing.
It seems to work and fail at random and I'm seeking help here.
The problem I am encountering is below.  When I call the handler directly in the browser, this is the error I see and when it works, I see nothing in the browser (as expected). 
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