Open Method of workbooks class failed
 

I'm doing COM automation on Excel. When I run several excel
(eg. 50 excel.exe) concurrently in the background, I get
this error when opening a .xls file: "Open Method of
workbooks class failed". My program opens an excel file saves into HTML
and closes it. The error starts after about 25 runs
Any ideas why I get this error only when running excels
concurrently in the background ?