Horde webmail login page shows the following error:
404: file or directory not found. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.
Webmail for the domain is not specified.
Set the webmail client for domain in Plesk: Home > Domains > example.com > Mail > Change Settings > Webmail > Horde.