That message can be the result of erasing your eBay cookies, or of your browser not saving cookies properly, or automatically erasing cookies between sessions. Using a private or incognito browser window, or using a new browser or a new device, or using certain "cleaner" programs that can erase browser cookies periodically can also cause this.
If your browser has a corrupt cookie, manually erasing your eBay cookies once may help.
It can also be caused by using a new network connection, or by using a VPN or a hotspot, or certain kinds of privacy software.
