FIXED: Linux sync client no longer working

FIXED: This issue was related to my web server configuration, which somehow broke with the latest update. (Using the latest Caddyfile provided in the official Caddy repo works)

Nextcloud version: 11.0.2
Operating system and version: Ubuntu 16.04.01
Apache or nginx version: Caddy
PHP version: 7.0.15
Is this the first time you’ve seen this error?: Since 11.0.2. and even after reinstalling a clean version

Can you reliably replicate it? (If so, please outline steps):
Delete everything, re-install Nextcloud, use Linux desktop client.

The issue you are facing:
I cannot sync my files using the Linux desktop client. The Windows client works fine for me, but the Linux client throws me Service Unavailable (could not get directory list) all the time.
Even after re-installing Nextcloud (deleting database, clearing data directory, removing web files), I still get the error on Linux.

My log files are all empty.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
Cannot access it right now, but I did not modify anything.

My data directory is a Samba share but it’s fully readable and writeable.

I think that this issue started occuring after updating to 11.0.2.