Me too!
Office documents and PDF are also not working.
I have installed Preview Generator 2.1.0 according to:
It worked until Nextcloud 14 on Ubuntu 16.04
While executing the Preview Generator cronjob i have this error:
(process:13186): dconf-CRITICAL **: 10:40:06.399: unable to create directory '/var/www/clients/client1/web7/.cache/dconf': Operation not permitted. dconf will not work properly.
func=xmlSecCheckVersionExt:file=xmlsec.c:line=188:obj=unknown:subj=unknown:error=19:invalid version:mode=abi compatible;expected minor version=2;real minor version=2;expected subminor version=25;real subminor version=26
I have filed a bug as well but no answer (yet) @rullzer