Any suggestions?
Yes, for some reason, fopen failed to open the file. There can be many reasons, the most likely being that the file doesn't exist but other reasons are possible (permission problem, etc.)
You never check that the fopen succeeds which is never a good idea.
[fid, errmsg] = fopen(filename, mode);
if fid < 0
error('Failed to open "%s", the error message was: %s');
end
%...
would at least tell you what the problem is.
