I'm a new user (installed last week) and just upgraded from 1.2.14.2 to 1.2.15
(In 1.2.14.2 I imported an opml file, but only a few feeds would update, claiming an error in most of the feeds, which updated just fine on another system using SimplePie?.)
Following the upgrade according to the instructions, tt-rss.php would not load properly. I found multiple require_once calls to config.php and functions.php as well as others; some calls were to ../filename as well as calls to the file in the same directory. I fixed this up to call the files from the proper location.
Now I get calls to undefined functions, beginning with login_sequence() It makes no difference whether I'm in single or multi-user mode (multi is my 'correct' setting). I tried commenting out the call to that function temporarily, and just get another one further on, e.g.:
Fatal error: Call to undefined function: login_sequence() in ...tt-rss.php on line 26
Fatal error: Call to undefined function: getfeedunread() in ...functions.php on line 39
Fatal error: Call to undefined function: __() in ...sanity_check.php on line 16
Fatal error: Call to undefined function: getfeedunread() in ...functions.php on line 48
(Sorry about the codeblock for non-code, just trying to escape the wiki underline caused by one of the function names.)