[solved] Logged in users appear logged out when viewing the front page

I had previously tried the authcache module, but decided to not use it, so I disabled it but forgot to remove the line in the settings.php: $conf['cache_inc'] = './sites/all/modules/authcache/authcache.inc';

At some point I realized that logged in users appeared to be logged out when viewing the front page, and could not understand why. Navigating to any other page showed that this this was clearly not the case, they were still logged in.

If I disabled the ordinary Drupal cache the problem went away. I tried everything; clearing the cache, page cache, even the watchdog table, and some other stuff, but no cigar.

It wasn't until I decided to do some Drupal cleanin' and deleted the authcache module from the modules that the whole web site stopped working, that I finally realized that the culprit might be the authcache line in the settings.php

It is now gone, and so so is the "logged in users appear logged out on the front page" problem.