Quote from Aliia ca47, following steps should do the trick for you. Please note, that as I told before this is custom code modification and what you do,  you do at your own risk:

1. ow_system_plugins/base/controllers/join.php
find following piece of code :
  if ( $this->displayAccountType )
        {
            $joinAccountType = new Selectbox('accountType');
            $joinAccountType->setLabel(OW::getLanguage()->text('base', 'questions_question_account_type_label'));
            $joinAccountType->setRequired();
            $joinAccountType->setOptions($accounts);
            $joinAccountType->setValue($this->accountType);
            $joinAccountType->setHasInvitation(false);

            $this->addElement($joinAccountType);
        }

And comment out ( using /* and */ ) this line: $this->addElement($joinAccountType); 

So you should have:

  if ( $this->displayAccountType )
        {
            $joinAccountType = new Selectbox('accountType');
            $joinAccountType->setLabel(OW::getLanguage()->text('base', 'questions_question_account_type_label'));
            $joinAccountType->setRequired();
            $joinAccountType->setOptions($accounts);
            $joinAccountType->setValue($this->accountType);
            $joinAccountType->setHasInvitation(false);

            /*$this->addElement($joinAccountType); */
        }

2. After that comment out ( using {* and *} ) the code bellow in ow_system_plugins/base/views/controllers/join_index.html :

 {* {if $displayAccountType==true}
            {<tr class="{cycle values='ow_alt1,ow_alt2'} ow_tr_first ow_tr_last">
                <td class="ow_label">
                    {label name='accountType'}
                </td>
                <td class="ow_value ow_center">
                    {input name='accountType'}
                    <div style="height:1px;"></div>
                    {error name='accountType'}
                </td>
                <td class="ow_desc">
                    {question_description_lang name="accountType"}
                </td>
            </tr>
            <tr class="ow_tr_delimiter"><td></td></tr>
            {/if} *}

If i do believe this variable: private $displayAccountType = false. Is to show Accounttype in the User config. So users can change it themselves. The solution does work since i have done it myself and is working on my website.

includes/config.php

Dev mode: true

Delete all cached files from smarty cache and then you will see the errors are gone. 

Let me know if this solved you problem.

LOL, No worries. We all have this moment when stress is taking over XD.