I'm getting an error with the xlswrite command

Excel returned: Error: Object returned error code: 0x800A03EC.

Thie first time I ever got that error I fixed it by changing the file name from filename.xls to filename.xlsx

But now I don't know what to do since I already have the files set to .xlsx

   U = y(:,1).*phi1(1) + y(:,3).*phi2(1) + y(:,5).*phi3(1)+ y(:,7).*phi4(1) + y(:,9).*phi5(1);
  V = y(:,2).*phi1(1) + y(:,4).*phi2(1) + y(:,6).*phi3(1)+ y(:,8).*phi4(1) + y(:,10).*phi5(1);
  figure
  plot(t,U)
  title('U plot')
  figure

plot(t,V)
title('V plot')
filename = strcat('2T_twicenatural','_sig',num2str(sig),'.xlsx');
count=1;
Table=horzcat(t,U);

xlswrite(filename,Table);

This is part of the code that I'm using. When I plot it the information comes out but for somereason it doesn't want to save in an excel file.

[Walter Roberson]

How large does your array Table come out as?

[Valeria Garcia]

It's pretty large usually around 20,000 numbers. Most of the times it converts it to .csv