Update: Clue
I installed Processwire via MyWebApp, and received the same error as in the processwire package. This tells me it’s an issue of nginx config for Processwire specifically on Yunohost.
Update: Success!
Changes in the following files fixed this issue…
Change 1
File path & name: /var/www/processwire/.htaccess
Added this line:
RewriteBase /var/www/processwire
This shouldn’t really matter because nginx ignores it, but I’m listing everything I changed, in case it helps someone else (and perhaps my future self, too). 
Change 2
File path & name: /etc/nginx/conf.d/mydomain.tld.conf
Current (working) config:
map $http_upgrade $connection_upgrade {
default upgrade;
'' close;
}
server {
listen 80;
listen [::]:80;
server_name mydomain.tld www.mydomain.tld;
index index.php index.html;
root /var/www/processwire;
charset utf-8;
# Common parameter to increase upload size limit in conjunction with dedicated php-fpm file
client_max_body_size 100M;
}
server {
listen 443 ssl http2;
listen [::]:443 ssl http2;
server_name mydomain.tld www.mydomain.tld;
index index.php index.html;
root /var/www/processwire;
charset utf-8;
# Common parameter to increase upload size limit in conjunction with dedicated php-fpm file
client_max_body_size 100M;
# -----------------------------------------------------------------------------------------------
# Processwire nginx rules
#
# -----------------------------------------------------------------------------------------------
# GLOBAL REWRITE PART 1
# Make nginx rewrite rules
try_files $uri $uri/ index.php;
# -----------------------------------------------------------------------------------------------
# Yunohost default rules
#
# -----------------------------------------------------------------------------------------------
include /etc/nginx/conf.d/security.conf.inc;
ssl_certificate /etc/yunohost/certs/mydomain.tld/crt.pem;
ssl_certificate_key /etc/yunohost/certs/mydomain.tld/key.pem;
more_set_headers "Strict-Transport-Security : max-age=63072000; includeSubDomains; preload";
# OCSP settings
ssl_stapling on;
ssl_stapling_verify on;
ssl_trusted_certificate /etc/yunohost/certs/mydomain.tld/crt.pem;
resolver 1.1.1.1 9.9.9.9 valid=300s;
resolver_timeout 5s;
location ^~ '/.well-known/autoconfig/mail/' {
alias /var/www/.well-known/mydomain.tld/autoconfig/mail/;
}
access_by_lua_file /usr/share/ssowat/access.lua;
include /etc/nginx/conf.d/mydomain.tld.d/*.conf;
include /etc/nginx/conf.d/yunohost_sso.conf.inc;
include /etc/nginx/conf.d/yunohost_admin.conf.inc;
include /etc/nginx/conf.d/yunohost_api.conf.inc;
include /etc/nginx/conf.d/yunohost_http_errors.conf.inc;
access_log /var/log/nginx/mydomain.tld-access.log;
error_log /var/log/nginx/mydomain.tld-error.log;
}
Change 3
File path & name: /etc/nginx/conf.d/drkj.au.d/processwire.conf
Current (working) contents of this file:
# sub_path_only rewrite ^/$ / permanent;
location / {
# Path to source
alias /var/www/processwire/;
# -----------------------------------------------------------------------------------------------
# Processwire nginx rules
#
# -----------------------------------------------------------------------------------------------
# DENY ACCESS TO EVERYTHING EXCEPT SSL CHECK
# Block access to any file or directory that begins with a period
location ~ /\.(?!well-known).* {
deny all;
}
# GIVE CONTROL TO PROCESSWIRE
# If the request is for a static file, then set expires header and disable logging.
# Give control to ProcessWire if the requested file or directory is non-existing.
location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|eot|woff|ttf)$ {
expires 15d;
log_not_found off;
access_log off;
try_files $uri $uri/ /index.php?it=$uri&$query_string;
}
# GLOBAL REWRITE PART 2
# Pass the PHP scripts to FastCGI server on local socket
location ~ [^/]\.php(/|$) {
fastcgi_split_path_info ^(.+?\.php)(/.*)$;
fastcgi_pass unix:/var/run/php/php8.2-fpm-processwire.sock;
fastcgi_index index.php;
include fastcgi_params;
fastcgi_param REMOTE_USER $remote_user;
fastcgi_param PATH_INFO $fastcgi_path_info;
fastcgi_param SCRIPT_FILENAME $request_filename;
}
# Amplify dashboard
location /nginx_status {
stub_status on;
allow 127.0.0.1;
deny all;
}
location = /favicon.ico { access_log off; log_not_found off; }
location = /robots.txt { access_log off; log_not_found off; }
error_page 404 /index.php;
# Include SSOWAT user panel.
include conf.d/yunohost_panel.conf.inc;
}
@ericg I am new to contributing code to projects, but if you would like n00b help, I am happy to do tasks to add working a nginx config to the processwire YNH package. Please let me know if you’d like help. 
Edit: The login page appears, but I have not been able to successfully log in using the details I entered during the Processwire setup 
Edit: I found more information about Processwire nginx config in in this gist: ProcessWire nginx configuration; Based on https://processwire.com/talk/topic/275-processwire-on-nginx/?do=findComment&comment=30534 · GitHub
Currently looking for clues here.
And more potentially useful config here: https://howtoforge.com/running-processwire-on-nginx-lemp-on-debian-wheezy-ubuntu-13.04