Fabbro, Apple has not acknowledged this bug and there is no fix yet, unless clearing your image cache works for you. For me, it had no effect. Turning off 3rd party plug ins did not fix it either.
Last week, I cleared all my tabs and started fresh. It took about a week for the problem to return (with around 30 tabs left open, day to day). Everyday, on restart, it seems to leave more and more blank. It's random so far.
All you can do is find them in history, (where they are saved) and reload them. If you have more than a few, it's a PIA, but it works.
Saxman, my safari retains the window title and displays this on the tab itself (truncated). If you see it too, just hold your mouse over the tab for a bit and it will display the whole title. The titlebar of your open window shows it as well. Search your browsing history using key words from that title and you will be able to locate the correct URL and reload the page. Almost certainly it will be in yesterday's history folder. If you discover a faster method, let us know.
If anyone wants to cheat this bug, save your healthy tabs as a folder of bookmarks and reload them all at once, should any go blank.
Before shutting down Safari, right-click on one of tabs and select "Add Bookmark for these "x" Tabs". Save each open page of tabs to the same bookmark folder and you'll have them ready to reload in a flash.
There was an Apple software update posted yesterday for security and safari that might resolve this issue, though I'm skeptical.