E-Mail-Postfach konnte nicht geƶffnet werden - Mail App mit Telekom

Hallo zusammen,

ich habe meinen Telekom-Mail Account in meiner Nextcloud Installation eingebunden. Das hat soweit auch alles ganz gut geklappt.
Allerdings habe ich das Problem, dass ich meinen Posteingang nicht ƶffnen kann. Die im Posteingang erstellten Unterordner werden angezeigt und die Mails kƶnnen auch geƶffnet werden.
(Unterordner sind im Posteingang angelegt)

Folgende Fehlermeldung erhalte ich:
{"reqId":"Kxxxxxxxxxxxxxxxxxxxxx","level":3,"time":"2020-11-13T23:44:16+00:00","remoteAddr":"79.xxx.xx.50","user":"user","app":"index","method":"GET","url":"/index.php/apps/mail/api/messages?mailboxId=27&limit=20","message":{"Exception":"TypeError","Message":"iconv() expects parameter 3 to be string, null given","Code":0,"Trace":[{"file":"/home/users/username/www/meineinstallation.de/apps/mail/lib/Address.php","line":78,"function":"iconv"},{"file":"/home/users/username/www/meineinstallation.de/apps/mail/lib/Address.php","line":68,"function":"getEmail","class":"OCA\\Mail\\Address","type":"->"},{"file":"/home/users/username/www/meineinstallation.de/apps/mail/lib/Address.php","line":93,"function":"getLabel","class":"OCA\\Mail\\Address","type":"->"},{"file":"/home/users/username/www/meineinstallation.de/apps/mail/lib/AddressList.php","line":103,"function":"jsonSerialize","class":"OCA\\Mail\\Address","type":"->"},{"function":"OCA\\Mail\\{closure}","class":"OCA\\Mail\\AddressList","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/home/users/username/www/meineinstallation.de/apps/mail/lib/AddressList.php","line":104,"function":"array_map"},{"file":"/home/users/username/www/meineinstallation.de/apps/mail/lib/Db/Message.php","line":255,"function":"jsonSerialize","class":"OCA\\Mail\\AddressList","type":"->"},{"function":"jsonSerialize","class":"OCA\\Mail\\Db\\Message","type":"->"},{"file":"/home/users/username/www/meineinstallation.de/lib/public/AppFramework/Http/JSONResponse.php","line":73,"function":"json_encode"},{"file":"/home/users/username/www/meineinstallation.de/lib/private/AppFramework/Http/Dispatcher.php","line":123,"function":"render","class":"OCP\\AppFramework\\Http\\JSONResponse","type":"->"},{"file":"/home/users/username/www/meineinstallation.de/lib/private/AppFramework/App.php","line":152,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->"},{"file":"/home/users/username/www/meineinstallation.de/lib/private/Route/Router.php","line":308,"function":"main","class":"OC\\AppFramework\\App","type":"::"},{"file":"/home/users/username/www/meineinstallation.de/lib/base.php","line":1009,"function":"match","class":"OC\\Route\\Router","type":"->"},{"file":"/home/users/username/www/meineinstallation.de/index.php","line":37,"function":"handleRequest","class":"OC","type":"::"}],"File":"/home/users/username/www/meineinstallation.de/apps/mail/lib/Address.php","Line":78,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0","version":"20.0.1.1","id":"5faf1a50ef4d7"}