Hi Paul,
Sorry for the delay in responding.
Error Occur when open the existing file with Footer:I am not able to reproduce the issue that you mentioned here. Also, I have created a simple sample to test this issue and it is available in the following link:
http://websamples.syncfusion.com/samples/XlsIO.Windows/F70858/main.htmPlease have a look at the above sample and if still the issue exists, could you please try reproducing it in the above sample and send us the modified sample with your template file so that we could sort out the cause of the issue and provide you a solution?
Regards,
Yavana.G