richdocumentscode: Blank page and "loolmount incorrect user-name, other than 'lool'"

Installed: Ubuntu 18.04 Nextcloud 21.0.3 PHP 7.4 Apache2 2.4.48 Collabora Online (richdocuments) 4.1.2 Collabora Online CODE 6.4.1003

When i start a Nextcloud, go to App Store and install the two Collabora apps, everything is fine. In the settings i see a “Connected”

But as soon as i want to open a document (or to reload my view of the document) the following output is generated:

web_1  | ==> /var/log/apache2/error.log <==
web_1  | [Mon Jul 12 13:52:51.973187 2021] [php7:notice] [pid 1147] [client 127.0.0.1:57640] richdocumentscode (proxy.php) error exit, PID: 1147, Message: Timed out opening local socket: 99 - Cannot assign requested address
web_1  | Security: loolforkit incorrect user-name, other than 'lool'
web_1  | Security: Check for the 'lool' username overridden on the command line.
web_1  | Init vcl
web_1  | preload: merged unordf ucpchelp1 msforms vbaobj pcr vbaswobj sw animcore hwp flash chartcore solver sc wpftcalc xof ucpcmis1 wpftdraw sd svgfilter evtatt graphicfilter wpftimpress sdfilt sm pdffilter PresentationMinimizer protocolhandler ucpdav1 wpftwriter msword writerfilter t602filter xmlfa basctl binaryurp uuresolver scd chartcontroller ldapbe2 dba sdbt deploymentgui migrationoo2 migrationoo3 xsltfilter sdd embobj emboleobj log expwrap odfflatxml textfd storagefd xmlfd frm fwl fwm io textconversiondlgs smd mozbootstrap oox scfilt slideshow proxyfac vclcanvas canvasfactory mtfrenderer simplecanvas dlgprov basprov stringresource dbaxml calc dbase flat writer reflection bootstrap introspection invocation invocadapt namingservice stocservices cached1 ucppkg1 srtrs1 ucptdoc1 xsltdlg swd cui bib guesslang numbertext offacc scn scriptframe dbpool2 xmlsecurity analysis date pricing fps_office i18nsearch xsec_xmlsec emfio vbaevents pdfimport mysql_jdbc ucpext hyphen spell lnth for ctl passwordcontainer svgio
web_1  | Absent (often optional): sm sm sm sm sm sm sm sm sm sm sm smd
web_1  | Disabled: ucpftp1 rptxml dbu rptui rpt mork odbc sdbc2 cmdmail syssh ucphier1 ucpimage PresenterScreen abp dbp updatefeed
web_1  | Whitelisted languages: de_DE el en_GB en_US es_ES fr_FR hu it nl pt_BR pt_PT ru
web_1  | Preloading dictionaries: de_DE en_US fr_FR it_IT nl_NL pt_BR [Mon Jul 12 13:52:54.588700 2021] [php7:warn] [pid 1145] [client 172.25.0.1:61196] PHP Warning:  file_get_contents(http://localhost:9983/hosting/capabilities): failed to open stream: HTTP request failed! in /var/www/nextcloud/apps/richdocumentscode/proxy.php on line 260
web_1  | ru_RU en_GB nl_BE pt_PT es_ES
web_1  | Preloading thesauri: de_DE en_US fr_FR it_IT nl_NL pt_BR ru_RU en_GB nl_BE pt_PT es_ES
web_1  | Preload icons
web_1  | Preload languages
web_1  | Preload fonts
web_1  | Ready to accept connections on port 9983.
web_1  |
web_1  |
web_1  | ==> /var/log/nextcloud.log <==
web_1  | {"reqId":"oU411yb5DWbdjVHA2kYL","level":2,"time":"2021-07-12T13:53:00+00:00","remoteAddr":"172.25.0.1","user":"admin","app":"no app in context","method":"GET","url":"/index.php/core/preview?fileId=101&c=60ec46878b41c&x=250&y=250&forceIcon=0&a=0","message":"Host localhost was not connected to because it violates local access rules","userAgent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.1.1 Safari/605.1.15","version":"21.0.3.1"}
web_1  | {"reqId":"MSP4fR2bpi7Wh7qzVYG8","level":2,"time":"2021-07-12T13:53:00+00:00","remoteAddr":"172.25.0.1","user":"admin","app":"no app in context","method":"GET","url":"/index.php/core/preview?fileId=88&c=60ec466f7ba42&x=250&y=250&forceIcon=0&a=0","message":"Host localhost was not connected to because it violates local access rules","userAgent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/14.1.1 Safari/605.1.15","version":"21.0.3.1"}
web_1  |
web_1  | ==> /var/log/apache2/error.log <==
web_1  | Security: loolmount incorrect user-name, other than 'lool'
web_1  | Aborting.
web_1  | Security: loolmount incorrect user-name, other than 'lool'
web_1  | Aborting.
web_1  | Security: loolmount incorrect user-name, other than 'lool'
web_1  | Aborting.
web_1  | [Mon Jul 12 13:53:33.365271 2021] [php7:notice] [pid 157] [client 172.25.0.1:61474] richdocumentscode (proxy.php) error exit, PID: 157, Message: No content in reply from loolwsd. Is SSL enabled in error ?

Followed by

web_1  | ==> /var/log/apache2/error.log <==
web_1  | Security: loolmount incorrect user-name, other than 'lool'
web_1  | Aborting.
web_1  | Security: loolmount incorrect user-name, other than 'lool'
web_1  | Aborting.
web_1  | Security: loolmount incorrect user-name, other than 'lool'
web_1  | Aborting.

with every try to open a document. It is running inside of docker container and the ports 80 and 443 are open.

What can i do? 😦