URL changes to: https://(null)/welcome

Hello,

i installed onlyOffice with this tutorial.

I also can not type in the url in the onlyoffice app at nextcloud. Then i get

Error while downloading the document file to be converted.

Now i get this errors:

When I go to the URL https://office.mysite.de. I get an error and the URL changes to: https://(null)/welcome
When I go to https://office.mysite.de/healthcheck It returns ‘True’ in the browser and the URL stays intact.

My SSL.conf:

<VirtualHost *:443>
ServerName xx-office.dedyn.io

SSLCertificateFile /etc/letsencrypt/live/xx-office.dedyn.io/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/xx-office.dedyn.io/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf

SSLCipherSuite EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH
SSLProtocol All -SSLv2 -SSLv3 -TLSv1 -TLSv1.1
SSLCompression off
SSLHonorCipherOrder on

SetEnvIf Host “^(.*)$” THE_HOST=$1
RequestHeader setifempty X-Forwarded-Proto https
RequestHeader setifempty X-Forwarded-Host %{THE_HOST}e
ProxyAddHeaders Off

SSLProxyEngine on

ProxyPassMatch (.*)(/websocket)$ “ws://127.0.0.1:81/$1$2”
ProxyPass / “http://127.0.0.1:81/
ProxyPassReverse / “http://127.0.0.1:81