Request Header Or Cookie Too Large Out of the Blue

A couple of days ago I started receiving the following error on all the websites I am hosting on my dedicated server running nginx only hestia:

400 Bad RequestRequest Header Or Cookie Too Large

After some hours of research I figured out that I needed to increase the number of worker_connections and large_client_header_buffers.
I did exactly that, but it wasn’t being solved. Then I read on this forum that someone who was facing the same issue was able to bring back his sites after doing the same thing, but he also had to suspend each user and un-suspend them to make it work.
After doing so myself, the error was resolved. All websites were back to normal.

However, webmail.mydomain.com and mail.mydomain.com (for all domains) were still doing the same. I went it and did the same susp/unsusp trick and got them back to work.

Now I am facing my third issue which is phpmyadmin.
I am getting the following error whenever I try to access it:

The nginx error logs show the following:

[error] 1011957#1011957: *3893 upstream sent too big header while reading response header from upstream, client: 136.***.***.***, server: hestia.mydomain.com, request: "GET /favicon.ico HTTP/1.0", upstream: "https://136.***.***.***:443/favicon.ico", host: "hestia.mydomain.com", referrer: "https://hestia.mydomain.com/phpmyadmin/"

My current /etc/nginx/ngin.conf :

# Server globals
user                 www-data;
worker_processes     auto;
worker_rlimit_nofile 65535;
error_log            /var/log/nginx/error.log;
pid                  /run/nginx.pid;
include              /etc/nginx/conf.d/main/*.conf;
include              /etc/nginx/modules-enabled/*.conf;

# Worker config
events {
	worker_connections 2048;
	use                epoll;
	multi_accept       on;
}

http {
	# Main settings
	sendfile                        on;
	tcp_nopush                      on;
	tcp_nodelay                     on;
	client_header_timeout           180s;
	client_body_timeout             180s;
	client_header_buffer_size       8k;
	client_body_buffer_size         256k;
	client_max_body_size            1024m;
	large_client_header_buffers     4 16k;
	send_timeout                    60s;
	keepalive_timeout               30s;
	keepalive_requests              1000;
	reset_timedout_connection       on;
	server_tokens                   off;
	server_name_in_redirect         off;
	server_names_hash_max_size      512;
	server_names_hash_bucket_size   512;
	charset                         utf-8;
	# FastCGI settings
	fastcgi_buffers                 512 4k;
	fastcgi_buffer_size             256k;
	fastcgi_busy_buffers_size       256k;
	fastcgi_temp_file_write_size    256k;
	fastcgi_connect_timeout         30s;
	fastcgi_read_timeout            300s;
	fastcgi_send_timeout            180s;
	fastcgi_cache_lock              on;
	fastcgi_cache_lock_timeout      5s;
	fastcgi_cache_background_update on;
	fastcgi_cache_revalidate        on;
	# Proxy settings
	proxy_redirect                  off;
	proxy_set_header                Host $host;
	proxy_set_header                Early-Data $rfc_early_data;
	proxy_set_header                X-Real-IP $remote_addr;
	proxy_set_header                X-Forwarded-For $proxy_add_x_forwarded_for;
	proxy_pass_header               Set-Cookie;
	proxy_buffers                   256 4k;
	proxy_buffer_size               8k;
	proxy_busy_buffers_size         8k;
	proxy_temp_file_write_size      256k;
	proxy_connect_timeout           30s;
	proxy_read_timeout              300s;
	proxy_send_timeout              180s;
	# Log format
	log_format                      main '$remote_addr - $remote_user [$time_local] $request "$status" $body_bytes_sent "$http_referer" "$http_user_agent" "$http_x_forwarded_for"';
	log_format                      bytes '$body_bytes_sent';
	log_not_found                   off;
	access_log                      off;
	# Mime settings
	include                         /etc/nginx/mime.types;
	default_type                    application/octet-stream;
	# Compression
	gzip                            on;
	gzip_vary                       on;
	gzip_static                     on;
	gzip_comp_level                 6;
	gzip_min_length                 1024;
	gzip_buffers                    128 4k;
	gzip_http_version               1.1;
	gzip_types                      text/css text/javascript text/js text/plain text/richtext text/shtml text/x-component text/x-java-source text/x-markdown text/x-script text/xml image/bmp image/svg+xml image/vnd.microsoft.icon image/x-icon font/otf font/ttf font/x-woff multipart/bag multipart/mixed application/eot application/font application/font-sfnt application/font-woff application/javascript application/javascript-binast application/json application/ld+json application/manifest+json application/opentype application/otf application/rss+xml application/ttf application/truetype application/vnd.api+json application/vnd.ms-fontobject application/wasm application/xhtml+xml application/xml application/xml+rss application/x-httpd-cgi application/x-javascript application/x-opentype application/x-otf application/x-perl application/x-protobuf application/x-ttf;
	gzip_proxied                    any;
	# Cloudflare IPs
	include                         /etc/nginx/conf.d/cloudflare.inc;
	# SSL PCI compliance
	ssl_buffer_size                 1369;
	ssl_ciphers                     "ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:DHE-RSA-AES256-SHA256";
	ssl_dhparam                     /etc/ssl/dhparam.pem;
	ssl_early_data                  on;
	ssl_ecdh_curve                  auto;
	ssl_prefer_server_ciphers       on;
	ssl_protocols                   TLSv1.2 TLSv1.3;
	ssl_session_cache               shared:SSL:20m;
	ssl_session_tickets             on;
	ssl_session_timeout             7d;
	resolver                        127.0.0.53 valid=300s ipv6=off;
	resolver_timeout                5s;
	# Error pages
	error_page                      403 /error/404.html;
	error_page                      404 /error/404.html;
	error_page                      410 /error/410.html;
	error_page                      500 501 502 503 504 505 /error/50x.html;
	# Proxy cache
	proxy_cache_path                /var/cache/nginx levels=2 keys_zone=cache:10m inactive=60m max_size=1024m;
	proxy_cache_key                 "$scheme$request_method$host$request_uri";
	proxy_temp_path                 /var/cache/nginx/temp;
	proxy_ignore_headers            Cache-Control Expires;
	proxy_cache_use_stale           error timeout invalid_header updating http_502;
	proxy_cache_valid               any 1d;
	# FastCGI cache
	fastcgi_cache_path              /var/cache/nginx/micro levels=1:2 keys_zone=microcache:10m inactive=30m max_size=1024m;
	fastcgi_cache_key               "$scheme$request_method$host$request_uri";
	fastcgi_ignore_headers          Cache-Control Expires Set-Cookie;
	fastcgi_cache_use_stale         error timeout invalid_header updating http_500 http_503;
	add_header                      X-FastCGI-Cache $upstream_cache_status;

	# Cache bypass
	map $http_cookie $no_cache {
		default              0;
		~SESS                1;
		~wordpress_logged_in 1;
	}

	# File cache (static assets)
	open_file_cache                 max=10000 inactive=30s;
	open_file_cache_valid           60s;
	open_file_cache_min_uses        2;
	open_file_cache_errors          off;
	# Wildcard include
	include                         /etc/nginx/conf.d/*.conf;
	include                         /etc/nginx/conf.d/domains/*.conf;
}

I tried to suspend and unsuspend the database of all users (shot in the dark), but it didn’t work.

I would appreciate any help. Thank you.