The following fix worked for me on SME V5.5, try it on your SME 6.01 and plese post the result:
Edit line 75 of file: /home/httpd/html/horde/imp/templates/login/login.inc
original:
<td align="left"><input type="text" tabindex="1" name="imapuser" value="<?= Horde::getFormData('imapuser', getenv('REMOTE_USER')) ?>" /></td>
edited:
<td align="left"><input type="text" tabindex="1" name="imapuser" /></td>