if you mean the next cloud site, then you should use the admin user and password, that you entered when you installed next cloud.
Thats what I am using and it does nothing.
I tried thousands of configurations (Pebble’s too) and no luck here. I have to say that would like to access through truenas_certificate in order to get more security.
This trouble is driving me crazy and desperate.
Anyone with the solution?
Did you finally get it @pebble ?
Unfortunately no luck here aswell 
I tried @Benders solution and it worked as described (thank you for that) but as you said it is without https. If I “Edit” the working App installation from the truenas GUI and enable the default truenas certificate I get back to where I started with the issue.
Another thing I saw is that the IP of my system is out of range within the trusted_proxies list when installed with certificates. I tried editing it manually with nano (same as @Bender did) but after restarting the app from the GUI my change within that array is removed by the system. I might be completely wrong with this lead and I am not sure if this has something to do with the whole issue but this is where I am now.
I finally got it to work, but I’m sad to say that had to go back to Truenas 24.04 (dragonfish)
Not sure about where the problem comes from. It works smooth but whenever I update the system to Electric Eel, it automatically shows the untrusted issue.
As a conclusion… I’ll have to stay in Dragonfish
Timo -I did get Collabra to install and to open different documents. That is as far as I got before I had to go out of town.
@ Mr.TJS -
Not silly at all, I am beginner at this on the best of days. Good thing I am dogged about getting a working system becuase I see the value of it considering it is essential to my business.
Logging on the shell: go to Apps, select nextcloud, the right side column, scroll down there is I think 3 buttons, log, config, and a single symbol/button to open shell in Next cloud. Accessing Nextcloud shell there requires no admin login, I am pretty sure.
The other way: Goto the main TrueNas page, left column, settings, shell/console. Next, depending on how you set up the datasets(file folders for Nextcloud) will determine where that Config.php file is. Since Truenas has nano installed you can directly edit the Config file (I had no luck using searching using command line) you can edit it directly by CD to location of the config file and using sudo nano config.php to edit.
That just requires your FreeNas Admin or Root password. It will edit the config file either way. Make sure to save it.
@Prozac
I am not sure if it was the Proxies or the TrueNas certificate that was causing the issues. My goal was to be able to successfully login to Nextcloud. After that you may try testing if Proxies or the TrueNAS cert. was causing the issue.
If its the Truenas cert then you can always create a cert (free or paid) if its the proxies some testing to figure out how to setup properly or submit a bug report. It sounds like Pepple’s is the latter that the default cert is incompatible for his system.
@pebble
The system will reorganize and sometimes remove addresses or duplicates. I suspect from what you are writing that there is simply a problem with how we address proxies. Someone who is better understanding of code than me will have to help us at this point.
Hey bender, did you do anything special regarding collabora? I had the green checkmark in nextcloud, but always go an error “document could not be loaded”. In some other forums I read something about changing something in the WOPI allow list, but I couldn’t find any configuration for this