I have the same issue.

At some point, Opera "decides" to stop saving new bookmarks, the history, and the list of opened tab.

To a minimum, when there is a problem saving those parameters, Opera should work the user so we know that we should restart ASAP. As it stands, we have no clue and, at least in my case, it can be weeks later that I restart (or it crashes) and it means I'm losing all the bookmarks I've added (and those links are not going to be in my history either).