Deleting and re-inserting the images may solve this issue.
When your project is open, after clicking on the Save button (top right of the screen), you should see the following box appear: Options to save
If you're using images in your project, try to delete them and then re-upload them and save the project
If your project is able to save after deleting and re-uploading the images, convert the image file to a different file format. Then add the new image and attempt to save the project
You can also upload images directly to your account by clicking on the Files tab and selecting Upload File
When designing and adding the images to your project, search for the files saved in My Account
Try switching to a different internet browser - Chrome is the most compatible browser to use with Avery.com
If Chrome is already being used, try clearing your Browsing Data - beginning of time / close out of all pages / open a new page to make a new connection with our site (Project design will be lost)
You can try going to projects and instead of opening, select Apply a design to another product and use the same template number and see if that works.