Strange error with fopen

[Devel]

Hi all,
on my redhat 7.3 and kernel 2.4.22 i received a strange error from fopen in my application. When system is on heavy disk load the function:

picture=fopen(filename, "wb"); 

return null and  strerror(errno) print :No such file or directory.
The file is new so is natural the doesn't exist. Furthermore  more is the load of the disk more are this error message.
I can't understand this error message, any suggest?.