Pmwiki is pretty robust and can figure out and fix on its own lots of errors and config omissions.
However, if the wiki suddenly stops responding, try deleting the lock file (wiki.d/.flock)
When the error "Cannot modify header information -- headers already sent" is the first error message to appear, it almost always means that there are extra spaces or blank lines outside of the <?php ... ?> in one of the .php scripts being used.
<?php if (!defined('Pm Wiki')) exit();
If you set any passwords, and the wiki starts asking for a password for each page read or twice per edit, it usually indicates a problem with
Check (using phpinfo() or ?action=diag) whether sessions are enabled in your version of php, then check where the session.save_path is set to, and check the permissions on this directory and that session files are appearing there properly.
You might also try adding
to your local/config.php and see if that fixes things.
When editing pages, it is possible to continuously get the revision edit form (i.e. after pressing Save, the edit page comes back with
<<<<<<< stuff I just added plus older stuff ======= older stuff >>>>>>>
... placed around stuff you had just edited). It's a rare bug, but when it happens, a tested cure is to dowload the page from wiki.d using your normal ftp client, then immediately upload it.