CMS Made Simple Forums

Hi all,
Just doing some final testing before going live (version 1.0.2) and have discovered that if a user tries to register or change settings the following error is displayed:
Fatal error: Call to undefined function: lang() in /home/sites/mysite/public_html/cms/modules/FrontEndUsers/FrontEndUsers.api.php on line 814

I thought maybe this was something to do with an e-mail problem but CMS Mailer is working fine.
Any ideas?
Thanks in advance.

OK I seem to have cured a lot of the problems by installing the latest versions of the modules involved (which I should have done in the first place of course...)
Anyway, I'm now left with two problems, when a user tries to register I get the following error:
--Add Me - module:FrontEndUsers string:error_noresponsefromserver--

and if you click 'If you have already completed the registration form, but did not receive an email Click Here' this error is displayed:

Fatal error: Call to undefined function: _userdisplaylostregemailform() in /home/sites/energyfm.co.uk/public_html/cms/modules/SelfRegistration/action.default.php on line 411

TIA