... and does this randomly, both in frontend and backend, sometimes showing the correct pages, and rarely an error message saying, that "table '#__session' is marked as crashed and last (automatic?) repair failed".

After updating K2 to version 3.6.3, in the backend, when tried to edit one of items, I got this error:

Error decoding JSON data: Syntax error.

After upgrading to Joomla 3.6 I had a nasty surprise - in the Joomla Installer the tabs - Install from file,  Install from folder etc.., all but the Install from Web - where all gone. Of course, nothing found on the web: this happens when you are an early adopter. But I discovered the "fix" accidentally, on my own.

If you search on Google for the term "joomla can't login after moving to new server" - or something similar, you will get more, than 1 million hits for most of the combinations.

So, you aren't alone. And there are plenty of suggestions, on what might gone wrong and how you might find that, and there are verious fixes. But what if you moved the site to another domain? From development server to it's final place? This is a subject wich is rarely touched by those advices.

You just upgraded your Joomla site to latest Joomla 2.5 or 3.* and your regular, registered users cannot login to the frontend, seeing this error:

"You cannot access the private section of this site"

Today one of my clients called me with this problem:

Was a weird call, and proven to be a difficult problem