First off, thank you.
The problem, though: What you described is what I did ??
I deleted those files mentioned in the paragraph “Delete these Files and Folders”, just kept those mentioned in the “DO NOT DELETE these foldes and files” paragraph.
I also did quite the opposite – deleted all new files of the 2.1 update, then uploaded my back-up files – and still I’m confronted with that error.
As I mentioned, the error occurs when I try to run upgrade.php.