Hi
The open tabs are stored in the Bookmark.nsf.
Perhaps there is corruption in this db.
Try recreating the db by stopping Notes, rename the Bookmark.nsf file to OldBookmarkdb.nsf, delete the cache.ndk, and relaunch Notes. The Bookmark.nsf and Cache.ndk will automatically be recreated.
See if this resolves the problem.
If the problem persists, deeper investigation is required. I'd suggest opening a PMR and requesting the Support Engineer check if this is a match for an old SPR (ACGB7KFNSG) which documents the same problem - but that was back in v8.5 - it was closed back then as no plans to fix (insufficient weight)
Paul.