Mails are’nt going through and Email test isn’t working

Hi!
there are no emails delivered anymore.
This is the Error-Log from the Email test:

Versions:
WordPress: 4.9.2
WordPress MS: No
PHP: 5.6.33
WP Mail SMTP: 1.2.3

Params:
Mailer: smtp
Constants: No
ErrorInfo: SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
Host: smtp.strato.de
Port: 465
SMTPSecure: ssl
SMTPAutoTLS: bool(true)
SMTPAuth: bool(true)

Server:
OpenSSL: Yes
SMTP Debug:
2018-01-24 10:05:16 Connection: opening to ssl://smtp.strato.de:465, timeout=300, options=array (
)
2018-01-24 10:05:16 Connection: opened
2018-01-24 10:05:16 SERVER -> CLIENT: 220 smtp.strato.de ESMTP RZmta (P1 -)
2018-01-24 10:05:16 CLIENT -> SERVER: EHLO https://www.lebenshilfe-ostfalen.de
2018-01-24 10:05:16 SERVER -> CLIENT: 250-smtp.strato.de greets 2a01:238:4290:f600:e46b:47c5:1fad:3a51
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-DELIVERBY
250-SIZE 104857600
250-AUTH DIGEST-MD5 CRAM-MD5 LOGIN PLAIN
250-BURL imap
250-CHUNKING
250 HELP
2018-01-24 10:05:16 CLIENT -> SERVER: AUTH CRAM-MD5
2018-01-24 10:05:16 SERVER -> CLIENT: 334 PDliMDgwYWFlMjIwYWJlODE2MWJjYThmMDBjQHNtdHAuc3RyYXRvLmRlPg==
2018-01-24 10:05:16 CLIENT -> SERVER: bGhvZkBhbXRlbmJyaW5rLmRlIDM0ZmM3NWYwMmVhZDMwMGM0NzFlZjFmYzBiZThlODZj
2018-01-24 10:05:20 SERVER -> CLIENT: 535 5.7.8 Authentication failed: wrong user/password
2018-01-24 10:05:20 SMTP ERROR: Username command failed: 535 5.7.8 Authentication failed: wrong user/password
2018-01-24 10:05:20 SMTP Error: Could not authenticate.
2018-01-24 10:05:20 CLIENT -> SERVER: QUIT
2018-01-24 10:05:20 SERVER -> CLIENT: 221 2.0.0 closing connection
2018-01-24 10:05:20 Connection: closed
2018-01-24 10:05:20 SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting

Thank you for your help!

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