I run Vaultwarden latest version on StartOS.
Have tried from different webbrowsers and in the apps.
The file I am trying to upload is only 1 Mb. it is an .PDF file.
The sice limit says it is 500 Mb.
I’m not familiar with StartOS but it should be the same file where you specified the proxy_pass directive to your vaultwarden instance. Typically the nginx configuration is located in /etc/nginx and the config file should look something like this: