Hey not sure if this is still a problem for you, but an update to swag broke the same thing for me. Here's what I did to fix it:
Launch to web console from your server's webUI
Type the following commands:
cd /mnt/user/appdata/swag/nginx/proxy-confs
mv collabora.subdomain.conf collabora.subdomain.conf.old
cp collabora.subdomain.conf.sample collabora.subdomain.conf
Edit anything in the config file if needed (like if your subdomain isn't exactly collabora.your-domain.com or something)
Restart swag and you should be good to go!