Yes, just put username=xxx,password=yyy in the fstab options as in the original example. Just to be sure that the error doesn't come from the .smbcredentials file.
Also try to remove the sec option altogether to see if it changes anything. Basically, have the line in your fstab like this:
//personalcloud/deadlyhydra /media/windowsshare cifs username=xxx,password=yyy 0 0