Debian9 + Nextcloud 13 + Collabora online + Collabora nextcloud app

Hi All

I’ve a working Nextcloud 13 in Debian 9.
Now Im trying to install the collabora server + collabora nextcloud app.

I have already installed the collabora server and is listening in 9980 and 9981.
Also already configured apache with the virtual host as reverse proxy of the collabora server as documented. With letsencrypt test certificates (that are not trusted).

Also installed the plugin and configured the plugin with my collabora server url

But when accesing an “office” document, nextcloud shows the following:

Internal Server Error
The server was unable to complete your request.
If this happens again, please send the technical details below to the server administrator.
More details can be found in the server log.

I dont found any interesting log. /var/log/loolwsd.log is not generated. Error log in apache also does not show anything.

The call that generates the error message is:

How can i troubleshoot this?
I dont think it is related, but must be the certificate trusted?

Thanks all!!