We have a website with a menu in a block, that is placed into a region "Sidebar". My customer notified me that the whole region containing the menu keeps disappearing on the live site, on average once a week.
To quick-fix the problem, the customer goes into Structure -> Menu and re-saves the menu, then the menu reappears.
It's a pity that we can't reproduce the problem yet. When it occurred last time, we asked the customer to save the HTML source with the missing menu, and found out that not only the menu or the block but the whole region Sidebar is missing.
I suspect the problem to be somewhat caching-related. Any ideas where I could look more specifically?