There's an incompatibility with DreamHost's Domains “Extra Web Security” feature and DokuWiki.

With or without “extra security”, when you use the file tag, you can make a one-liner with PHP/HTML text like so:

<link href="<?php echo DOKU_TPL?>index.php" />

But if you turn on extra security and split the one-liner into two lines like so:

<link
 href="<?php echo DOKU_TPL?>index.php" />

You can't preview or save the change. DokuWiki will say “This topic does not exist yet” and redirect you to a “forbidden” page.