First let me say what a great job Chris did. He literally saved my business.
Now let me recap what he found..
An image file was missing from the images folder.
TCPDF ERROR: [image] Unable to get image: /home/oursite/public_html/whmcs/images/logo.png
I guess the Invoice PDF creator doesn't have a failsafe check on the existence of the image and stopped the invoice process when it could not find the image.
Be careful not to forget custom files,etc.. when moving and or upgrading. We had moved servers and during this we had upgraded versions of WHMCS so we did not move the old files and the image was left out.