I have the exact same problem.
Last night I made a backup before shutting down for the day, and today Opera v99 autoupdated to Opera One v100, and it wiped all the tabs and workspaces.
On Opera v99 it worked great, but every time I copy the backed up files files back into %appdata%\Opera Software\Opera Stable\Sessions I get all the tabs and workspaces back.
If I just close and reopen Opera, the tabs survive, but if I do anything, like click on a link or move a tab to a different workspace, the next time I open Opera One, all tabs are gone.
For now I can live with wiping the Sessions folder contents and copying back the backed up files, but I have like 100+ tabs, and the new version makes it much harder than normal to click to get the right tab coming to the front.