Unable to log into any mailbox via the webmail: Call to undefined method stdClass::write()

Symptoms

• Unable to log into any Plesk hosted mailbox using the webmail client, the error below is shown:
  

  CONFIG_TEXT: A fatal error has occurred
  Call to undefined method stdClass::write()
  Details have been logged for the administrator.

• There was an attempt to change the default location of mailboxes.
• The following error can be found in /var/log/maillog file:
  

  CONFIG_TEXT: service=imap, user=john.doe@example.com, ip=[::1]. Error: User initialization failed: Namespace 'INBOX.': mkdir(/var/qmail/mailnames/example.com/john.doe/Maildir) failed: Permission denied (euid=30(popuser) egid=31(popuser) missing +w perm: /var/qmail/mailnames, dir owned by 0:0 mode=0755)
  imap: Error: Invalid user settings. Refer to server log for more information.

Cause

Incorrect Dovecot configuration.

Resolution

1. Log into the server via SSH.
2. Complete the configuration of custom mailboxes location using the steps from the article: 
   • How to change the default location of mailboxes in Plesk for Linux?