Hi! I ran into the same problem and have been researching this issue. Apparently the problem is the version for Dovecot. From what I have read these instructions are for the previous version. However, I am curious as to how you got this to work. I followed your solution by pointed the conf file to .ext and also removed the lines you mentioned. At this point I got some success in running service dovecot restart. However something is still wrong I get the results below:
/var/log/mail.log and /var/log/mail.err are both empty
dovecot-deliver.log file under /home/vmail produces:
2013-03-25 13:05:25 lda: Fatal: Internal error occurred. Refer to server log for more information.
2013-03-25 13:15:25 lda: Error: userdb lookup(
myusername@mydomainname.com): Disconnected unexpectedly
telnet localhost pop3 results:
Connected to localhost.domainname.com.
Escape character is '^]'.
Connection closed by foreign host.
Any help in this matter would be greatly appreciated.