Vista - Why is my sessions suddently lost? Its happening in CreateTextFile

Hi

Suddently for a website I have been using for several years it started to
show me login screens all the time.
It happened because the sessions was lost.
I have now been tracing a lot and can reproduce the problem allmost 99% of
the times with a little testprogram.

Both the built in sessionID and my own session variable is lost in the way
that the sessionID is incremented by at least 1 and my own session cookie
just disappears!

I can make the sesison lost with this codeline:

set fileObj = fsoObj.CreateTextFile(filename)

Strangely I found out that if I ensures that the length of the filename
incl. path is below 86 chars, then it doesnt kill the session!
(But it have all worked before for several years!)

Then it continues to the next line:

set fileObj = fsoObj.getFile(filename)

And then this line:

set tsObj = fileObj.OpenAsTextStream(forWriting, tristateFalse)

And then it just kills the session here!

Strangely, then I have found out, if I change the length of the filename
again, this time below 61 chars, then it can also process this line without
killing my session.

What the heck is this? ANy ideas?

I have:
Win Web Server 2003
classic ASP

The problem can be reproduced in both IE 6 & 7 and FF, so it doesnt seem to
be client related.
Could it be:
- a windows update?
- a corrupted IIS?
- a IIS setting?
- users or user rights?

PS: The program is running and processing all codelines - it just kills the
sessionID and my own session cookie

It happened here in december 2008.