Security & setup warning "Error: Cannot connect to server"

[Anvil5465]

For several months now, nextcloud has been reporting errors about not being able to connect to itself. I'm running the most recent stable version of nextcloud aio on ubuntu 22.04 with community containers caddy, jellyfin, jellyseer, and fail2ban. I've tried the basic troubleshooting it recommends with no luck. I found a few threads with users who had similar issues, but their fixes did not work either. I also tried disabling all community containers, but then was unable to connect to the server at all. I've been having an issue with Nextcloud talk where I cannot make group chats which I believe may be related. Additionally, the server is able to ping itself if I manually run a ping command from the nextcloud-aio-nextcloud container. Any suggestions would be very much appreciated. Happy to provide any additional information.

There are some errors regarding your setup.

Error: Cannot connect to server

Could not check that the data directory is protected. Please check manually that your server does not allow access to the data directory. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule.
Your webserver is not set up to serve `.js.map` files. Without these files, JavaScript Source Maps won't function properly, making it more challenging to troubleshoot and debug any issues that may arise.
Unable to run check for JavaScript support. Please remedy or confirm manually if your webserver serves `.mjs` files using the JavaScript MIME type. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule.
Could not check if your web server properly resolves the OCM and OCS provider URLs. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule.
91 errors in the logs since July 26, 2025, 12:15:25 AM

Could not check that your web server is properly set up to allow file synchronization over WebDAV. Please check manually. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule. For more details see the [documentation ↗](https://docs.nextcloud.com/server/31/go.php?to=admin-setup-well-known-URL).
Could not check that your web server serves `.well-known` correctly. Please check manually. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule. For more details see the [documentation ↗](https://docs.nextcloud.com/server/31/go.php?to=admin-setup-well-known-URL).
Could not check for otf loading support. Please check manually if your webserver serves `.otf` files. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule. For more details see the [documentation ↗](https://docs.nextcloud.com/server/31/go.php?to=admin-nginx).
Could not check that your web server serves security headers correctly. Please check manually. For more details see the [documentation ↗](https://docs.nextcloud.com/server/31/go.php?to=admin-security).
You have not set or verified your email server configuration, yet. Please head over to the "Basic settings" in order to set them. Afterwards, use the "Send email" button below the form to verify your settings. For more details see the [documentation ↗](https://docs.nextcloud.com/server/31/go.php?to=admin-email).
No recording backend configured
No SIP backend configured
Could not check for WASM loading support. Please check manually if your web server serves `.wasm` files. To allow this check to run you have to make sure that your Web server can connect to itself. Therefore it must be able to resolve and connect to at least one of its `trusted_domains` or the `overwrite.cli.url`. This failure may be the result of a server-side DNS mismatch or outbound firewall rule. For more details see the [documentation ↗](https://docs.nextcloud.com/server/31/go.php?to=admin-nginx).