See: https://github.com/ONLYOFFICE/DocumentServer/issues/888

Thank you CSchmidtDD Christof T. https://github.com/CSchmidtDD

InstallConfig's Note: ds:example service is NOT disabled by default, at least not in my installation on Ubuntu using the community-edition, and, apparently it is also NOT disabled when using the enterprise-edition installation guide.

CSchmidtDD's Comments: And even if it were disabled, it messes up with the nginx config by having the link to the ds-example.conf in there. The welcome page is messed up by default that way and should not be showing an “example” index.html, at least not for an “enterprise-edition.”

If you installed the Document Server (any edition) and have the same problem - that being, the welcome page was the example index.html, then open a terminal or ssh remotely into your ubuntu or debian server and take the following steps offered by CSchmittDD to get rid of the example:

rm -rf /etc/onlyoffice/documentserver-example/  #Removes the whole unnessecary "example"
rm /etc/nginx/includes/ds-example.conf   #This will delete the link that stops nginx from starting with error like you mentioned
service nginx restart   #Should restart without problem
service supervisord restart  #As Example-Service is gone, it will re-register to only four active packets 
supervisorctl restart docservice   #Restart docservice and now you can see the green checkmark

After these steps, the OnlyOffice-Server should now show you a green checkmark on the welcome page that everything is running correctly.

Hope this fixes your problem(s).

During and after the Upgrade, it echoed that was something missing. Notice that, during the ubuntu upgrade of onlyoffice server, there was a warning that under /documentserver-example/ the default.json file had been deleted. The upgrade script prompted whether to keep my currently-installed version – select N (default) – or to install the package maintainer's version – Select Y. And, Y was chosen, which is probably why the Nginx server wouldn't start after the Upgrade, and why OnlyOffice Server was offline and unavailable to NextCloud server.

The same happened for production-linux.json (YES), production-windows.json (YES), etc. So, it would seem that all that needs to be removed again.

Following is part of the Upgrade in this regard:

Setting up onlyoffice-documentserver (8.1.1-26) ...

Configuration file '/etc/onlyoffice/documentserver-example/default.json'
 ==> Deleted (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** default.json (Y/I/N/O/D/Z) [default=N] ? Y
Installing new version of config file /etc/onlyoffice/documentserver-example/default.json ...

Configuration file '/etc/onlyoffice/documentserver-example/production-linux.json'
 ==> Deleted (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** production-linux.json (Y/I/N/O/D/Z) [default=N] ? Y
Installing new version of config file /etc/onlyoffice/documentserver-example/production-linux.json ...

Configuration file '/etc/onlyoffice/documentserver-example/production-windows.json'
 ==> Deleted (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : start a shell to examine the situation
 The default action is to keep your current version.
*** production-windows.json (Y/I/N/O/D/Z) [default=N] ? Y
Installing new version of config file /etc/onlyoffice/documentserver-example/production-windows.json ...
Installing new version of config file /etc/onlyoffice/documentserver/default.json ...
Installing new version of config file /etc/onlyoffice/documentserver/development-linux.json ...
Installing new version of config file /etc/onlyoffice/documentserver/development-mac.json ...
Installing new version of config file /etc/onlyoffice/documentserver/development-windows.json ...
Installing new version of config file /etc/onlyoffice/documentserver/logrotate/ds.conf ...
Installing new version of config file /etc/onlyoffice/documentserver/nginx/ds-ssl.conf.tmpl ...
Installing new version of config file /etc/onlyoffice/documentserver/nginx/includes/ds-common.conf ...
Installing new version of config file /etc/onlyoffice/documentserver/nginx/includes/ds-docservice.conf ...
Generating WOPI private key...Done
Generating WOPI public key...Done
Generating AllFonts.js, please wait...Done
Generating presentation themes, please wait...Done
Generating js caches, please wait...Done
Installing plugins, please wait...Done
dpkg: error processing package onlyoffice-documentserver (--configure):
 installed onlyoffice-documentserver package post-installation script subprocess returned error exit status 1

sudo rm -rf /etc/onlyoffice/documentserver-example/  #Removes the whole unessecary "example"
sudo rm /etc/nginx/includes/ds-example.conf   #This will delete the link that stops Nginx from starting with error
sudo service nginx restart   #Should restart without problem