There were some news this year that Nextcloud is officially supported (Nextcloud now officially supported on TrueNAS) by TrueNAS. I found a link to an installation description with a link to a contact where they invite you to contact them in case of problems: https://www.ixsystems.com/blog/nextcloud-and-truenas-collaborate-to-help-you-build-your-own-private-cloud/. So perhaps ask them nicely if they have ideas how to proceed.
They know perhaps a few more steps to investigate.