Hey thanks for reporting this @ice.d! I just had another look myself and after clearing the cache on my phone and turning off wifi, I saw the same mailcow login!
With some help from AI, I learned that because my server has IPv6 my reverse proxy config has to listen for IPv6 as well as IPv4. Apparently mobile networks prefer IPv6.
server {
listen 80;
listen [::]:80; # ← add this
server_name discourse.tobiaseigen.org;
...
}
server {
listen 443 ssl;
listen [::]:443 ssl; # ← add this
server_name discourse.tobiaseigen.org;
...
}