Hi,
Well, since it seems to be something between either the database or the user session, one thing to do would be to try copying the website on another hosting and see if you still have the same issue (maybe try on a local web server?).
And also try using another template, or disabling extensions to see if anything is causing the problem.
Once identified, you can switch to something else or report the problem.
So it doesn't really require a specialist to find the issue for now, but I think it would be better to swap different elements to understand which one is causing the problem. If it's the hosting, you can just change the hosting and problem solved.
If it's an extension, maybe there is an alternative ?
And if not, then you can check with a developer specialized in the subject.