Hi Mike,
Same error occurred here also after my hosting company switched over to Litespeed.
I'm trying to solve it by changing the settings in CP panel for error document 404 to redirect to /page-not-found instead of 404.php
Let you know as soon as server updated from config file
ErrorDocument 404 /page-not-found
Because I can as some one else in this thread reach "/pagenotfound" when friendly-url is on
The strange thing is that the table "pagenotfound" is populated with pages even if the page is not redirected correctly, seems to me like the error occurs when the 404.php tries to redirect to /page-not-found
In -htaccess it says
RewriteRule ^page-not-found$ /404.php [QSA,L]
But after trying and disabled friendly-url, emptied all cache and compile, it was still the same problem, with the difference that I could view the page "/404.php" that did not work when I had friendly-url on, because then "/page-not-found" worked
With firebug I get this
HTTP/1.1 404 Not Found
Content-Encoding: gzip
Vary: Accept-Encoding
Date: Thu, 08 Nov 2012 12:01:10 GMT
Server: LiteSpeed
Connection: close
X-Powered-By: PHP/5.2.17
Content-Type: text/html; charset=utf-8
Cache-Control: no-cache
Location: http://domain.yyy/page-not-found
Content-Length: 20
/BoA