Debian 9.2

Apache/2.4.25 (Debian)
Nextcloud 12.0.3
mkdir a folder for the script, clone or download the .zip
open officeonline-install.cfg with an editor and adapt the POCO parameters like ExaconAT shared it in the issue 135 (link above)
let the script run (started it w/o any parameters). It took about 80 min on my box.
I had to restart the script once because it complained over an inconsistency in /etc/group which needed to be fixed first. After restart the script managed to resume where it had stopped before.
created a subdomain for the lool (LibreOffice OnLine) to run in in DNS, a valid certificate for https, a virtual host config to tell apache how to proxy. Check that Apache has the required modules enabled:
proxy_wstunnel proxy proxy_http ssl
For the latter I used the example Nextcloud recommends in and this was a source of problems later. It took me hours of try and error & recherche to learn that all the ProxyPassReverse lines need to have a trailing slash at the end of the right hand argument. The relevant bit of the config I use is:
# Encoded slashes need to be allowed
AllowEncodedSlashes NoDecode

# Container uses a unique non-signed certificate
SSLProxyEngine On
SSLProxyVerify None
SSLProxyCheckPeerCN Off
SSLProxyCheckPeerName Off

# keep the host
ProxyPreserveHost On

# static html, js, images, etc. served from loolwsd
# loleaflet is the client part of LibreOffice Online
ProxyPass           /loleaflet retry=0
ProxyPassReverse    /loleaflet

# WOPI discovery URL
ProxyPass           /hosting/discovery retry=0
ProxyPassReverse    /hosting/discovery

# Main websocket
ProxyPassMatch "/lool/(.*)/ws$" wss://$1/ws nocanon

# Admin Console websocket
ProxyPass   /lool/adminws wss://

# Download as, Fullscreen presentation and Image upload operations
ProxyPass           /lool
ProxyPassReverse    /lool
The script creates a self signed certificate for lool in /etc/loolwsd
This is not helpful when things do not work at once because chrome and firefox are so very strict against self signed certs and any direct tests of the lool subdomain and the virtual host there are hindered. I got around this by changing the path to the certificate (cert, ca-chain privkey) to point at my valid cert by edit in /opt/online/loolwsd.xml. An increased log level here and file enable="true" are other useful settings here. 
Things look fine when https://lool.domain.tld:9980 results in an empty page with ok and https://lool.domain.tld:9980/hosting/discovery returns an xml of wopi discovery. If the latter gives a 500 then the proxy config may be the reason.
In nextcloud admin Collabora Online the entry is https://lool.domain.tld:9980 - during my recherche I found several discussions with recommendations to not include the port, but this only adds another error.
Sources of error messages included nextcloud admin logging (connection refused as long as the proxying doesnt work), /var/log/loolwsd.log (needs log level increased and file enabled) and of course the messages of systemctl status  loolwsd and systemctl status apache2
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