Template stuck at “Importing content”

Hi,
as written, when choosing any site template, after downloading plugins, I am getting an eternal:
“Importing content” with a spinning circle. Please troubleshoot.

thanks

server info:

Server architecture	Linux 6.8.0-62-generic x86_64
Web server	nginx/1.24.0
PHP version	8.3.22 (Supports 64bit values)
PHP SAPI	fpm-fcgi
PHP max input variables	10000
PHP time limit	600
PHP memory limit	768M
Max input time	600
Upload max filesize	256M
PHP post max size	512M
cURL version	8.5.0 OpenSSL/3.0.13
Is SUHOSIN installed?	No
Is the Imagick library available?	Yes
Are pretty permalinks supported?	Yes

WP constants:

ABSPATH	/var/www/clients/client0/web3/web/
WP_HOME	Undefined
WP_SITEURL	Undefined
WP_CONTENT_DIR	/var/www/clients/client0/web3/web/wp-content
WP_PLUGIN_DIR	/var/www/clients/client0/web3/web/wp-content/plugins
WP_MEMORY_LIMIT	768M
WP_MAX_MEMORY_LIMIT	768M
WP_DEBUG	Enabled
WP_DEBUG_DISPLAY	Enabled
WP_DEBUG_LOG	Disabled
SCRIPT_DEBUG	Disabled
WP_CACHE	Disabled
CONCATENATE_SCRIPTS	Undefined
COMPRESS_SCRIPTS	Undefined
COMPRESS_CSS	Undefined
WP_ENVIRONMENT_TYPE	Undefined
WP_DEVELOPMENT_MODE	Disabled
DB_CHARSET	utf8
DB_COLLATE	Empty value

wp-config extras:

define('WP_HTTP_TIMEOUT', 300);
define('WP_MEMORY_LIMIT', '768M');
define('DISABLE_WP_CRON', false);

ini_set('default_socket_timeout', 300);
define('WP_HTTP_TIMEOUT', 300);

php diretives:

memory_limit = 768M
max_execution_time = 600
max_input_time = 600
max_input_vars = 10000
max_input_nesting_level = 64
upload_max_filesize = 256M
post_max_size = 512M
max_file_uploads = 50
max_multipart_body_parts = 10000

opcache.enable = 1
opcache.enable_cli = 1
opcache.memory_consumption = 256
opcache.max_accelerated_files = 10000
opcache.max_wasted_percentage = 3
opcache.validate_timestamps = 1
opcache.revalidate_freq = 0
opcache.save_comments = 0
opcache.fast_shutdown = 1
opcache.enable_file_override = 1
opcache.interned_strings_buffer = 32

opcache.jit = 1255
opcache.jit_buffer_size = 128M
opcache.jit_hot_func = 127
opcache.jit_hot_loop = 64

realpath_cache_size = 8192K
realpath_cache_ttl = 1200

output_buffering = 8192
zlib.output_compression = Off
zlib.output_compression_level = 6

session.gc_probability = 1
session.gc_divisor = 100
session.gc_maxlifetime = 7200
session.cookie_httponly = 1
session.cookie_secure = 1
session.cookie_samesite = Lax
session.use_strict_mode = 1

display_errors = Off
log_errors = On
ignore_repeated_errors = On
expose_php = Off
allow_url_include = Off
allow_url_fopen = On

default_socket_timeout = 300
user_agent = ""
auto_detect_line_endings = Off

date.timezone = Europe/Berlin

mysqli.allow_persistent = On
mysqli.max_persistent = 50
mysqli.max_links = 50
mysqli.default_socket = /var/run/mysqld/mysqld.sock
mysqli.allow_local_infile = Off
mysqli.reconnect = Off

pcre.jit = 1
pcre.backtrack_limit = 5000000
pcre.recursion_limit = 500000

assert.active = Off
auto_globals_jit = On

error_reporting = 0
log_errors_max_len = 2048

mbstring.language = neutral
mbstring.encoding_translation = Off
mbstring.substitute_character = none

user_ini.cache_ttl = 600
variables_order = GPCS
request_order = GP

nginx directives:

client_max_body_size 128M;

# Security: Protect sensitive files
location ~* wp-config\.php {
    deny all;
}
location ~* /\.ht {
    deny all;
}

# WordPress Admin AJAX - NO CACHING (fixes import stalling)
location ~ ^/wp-admin/admin-ajax\.php$ {
    try_files $uri /index.php?$args;
    add_header Cache-Control "no-cache, no-store, must-revalidate" always;
    add_header Pragma "no-cache" always;
    expires -1;
}

# WordPress REST API - NO caching for POST requests (fixes imports)
location ~ ^/wp-json/ {
    try_files $uri $uri/ /index.php?$args;
    
    # Don't cache POST/PUT/DELETE requests (used by imports)
    set $no_cache 0;
    if ($request_method ~ ^(POST|PUT|DELETE|PATCH)$) {
        set $no_cache 1;
    }
    
    add_header Cache-Control "no-cache, no-store, must-revalidate" always if ($no_cache = 1);
    add_header Cache-Control "public, max-age=300" always if ($no_cache = 0);
    expires -1 if ($no_cache = 1);
    expires 5m if ($no_cache = 0);
}

# Static assets with aggressive long-term caching
location ~* \.(css|js|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot|webp|avif|pdf|mp4|webm|mp3|zip|tar|gz)$ {
    expires 1y;
    add_header Cache-Control "public, immutable, max-age=31536000" always;
    add_header X-Cache-Status "STATIC" always;
    access_log off;
}

# WordPress uploads directory
location ~* ^/wp-content/uploads/ {
    expires 1y;
    add_header Cache-Control "public, max-age=31536000" always;
    access_log off;
}

# WordPress themes and plugins
location ~* ^/wp-content/(themes|plugins)/ {
    expires 1y;
    add_header Cache-Control "public, max-age=31536000" always;
    access_log off;
}

# All other requests (HTML pages, WordPress content)
location / {
    try_files $uri $uri/ /index.php?$args;
    add_header Cache-Control "public, max-age=1800" always;
    expires 30m;
}