I find this link, it seem the problem similar with me , but i still have no idea how to solve this problem :If i disable certificate, the error message like below:
View attachment 72522
Error when trying to connect (Error occurred in the document service: Error while downloading the document file to be converted.) (version 5.5.1.76)✖
please advice,
Thankyou & regards.

Certificate has expired for apps.nextcloud and nextcloud.com
Perhaps you have another error. Post details and logs.

Especially this post :

Certificate has expired for apps.nextcloud and nextcloud.com
Hi, with a Nextcloud 21.0.4 jail installation on a TrueNas server i had to replace the jail file /etc/ssl/cert.pem here are the commands: jls 1 sh sudo su mv /etc/ssl/cert.pem /etc/ssl/cert.pem.bkp curl https://curl.se/ca/cacert.pem >> /etc/ssl/cert.pem hope this help!

Please advice..
Regards,