Ludek,
I marked your answer above as the correct answer. I read the links you provided and decided to try the easiest solution first as follows:
4) HTTP 401.2 - Unauthorized: Logon failed due to server configuration reported in Internet Information Services logs.
Reported solutions:
1) In the RDCrptserver115.asp (for 11.5) and rptserver.asp (for 8.5) files, change the 1st occurence of the line:
response.addheader "CONTENT-LENGTH", lenb(temp)
to
response.addheader "CONTENT-LENGTH", lenb(temp + 0)
I did this with RDCrptserver10.asp and in about five minutes our reports were rendering correctly.
Thank you very much for the assistance. This was a life saver!
Jack