It was checked in advance whether the permission for the e-mail address registered in the webmailer was assigned and "Access denied" or "Something didn't work" is still displayed? Please note the following tip.
1.
Via the known way a "Open another mailbox...".
Enter the desired e-mail address from the function mailbox, but do not take the suggested entry directly, but click on Search directory.
Background:
If the directly displayed entry is taken, an entry from the personal address book may be taken. This then triggers the error.