If you post the url of the own forum immediately followed by a slash and dot, then the dot is moved before the slash, thereby breaking the url.

Example:

http://www.phpbb.com/community/.

http://www.phpbb.com/.

http://www.phpbb.de/.

Result:
http://www.phpbb.com/community./
http://www.phpbb.com/.
http://www.phpbb.de/.

As you see, it only happens with the forum root url, not with other urls.