Yeah, the chart needs modified whoever the maintainer is. I'd suggest submitting a bug report. This should definitely be built into the chart esp if they are eliminating webdav outside of the app.
If you are "brave" and want to play around with Apache directives, you might try a hostpath for /usr/local/apache2/conf/Includes in the container to some directory on Truenas where you can edit the Apache configuration. It won't matter if you do something wrong as you can always delete the app and install again, and clear your conf file.
In Apache, you typically add a directive such as:
LimitXMLRequestBody 67108864
Within a directory or virtualhost block. The number is the max size in bytes. You want to make sure this is well within memory limits.
I don't have the app as I don't use webdav. From a cursory review of the chart.
If you do get to adding the directive in the appropriate spot, you will have to restart the container to take effect. Or, restart apache within the container.
I do not see any bug report submitted for this so, I would suggest doing so and posting the link here so others can follow or like it.