Hi Tom, I had this issue as well and it took one of my sites offline. You may have a serious bug on your hands. The site was running php 7 and i get this error:
Warning: require_once(/home/sites/6a/7/7ec09406f7/public_html/wp-content/plugins/gp-premium/backgrounds/generate-backgrounds.php): failed to open stream: No such file or directory in /home/sites/6a/7/7ec09406f7/public_html/wp-content/plugins/gp-premium/gp-premium.php on line 59
Deleted the gp premium plugin and installing it again seemed to fix the issue. I also set php version to 7.3.