turn on logging to see what went wrong

$conf['enable_i_log'] = true;

Okay I find that I could ectend the PHP memeory limit here:
\dockers\piwigo\php\php-local.ini

Adding:
; Edit this file to override php.ini directives and restart the container

date.timezone = Europe/Paris
memory_limit=1024M

And now it can synch again!

Release 1.7.0 of PhpWebGallery turn off this restriction displaying an intermediate page before fateful max_execution_time. This page will be reloaded (after many seconds) until file listing.xml is complete.

$conf['max_execution_time'] = (5*ini_get('max_execution_time'))/6;
It's maximum time between the script start and the intermediate page display.

Default value is 5/6 of server max execution time.

It SHOULD NOT be modified without assking for advice.
It SHOULD be part of server time.
It MUST NOT be equal to 0.

Ok I am asking for advice :-)

Yes I found the PHP file in the docker path and set it to 300 sec.

Here for other people running docker on Unraid: \appdata\piwigo\php\php-local.ini adding this line:
memory_limit=1024M
max_execution_time=300

And then I got this new error:

arning:  [mysql error 1062] Duplicate entry '293' for key 'PRIMARY'

INSERT  INTO `piwigo_categories`
  (`id`,`dir`,`name`,`site_id`,`id_uppercat`,`uppercats`,`commentable`,`visible`,`status`,`rank`,`global_rank`)
  VALUES('293','2020-11','2020-11','1','285','32,285,293','true','true','private','8','7.18.8')
  , ('294','2020-10','2020-10','1','285','32,285,294','true','true','private','9','7.18.9')
  , ('295','2020-09','2020-09','1','285','32,285,295','true','true','private','10','7.18.10')
  , ('296','2020-08','2020-08','1','285','32,285,296','true','true','private','11','7.18.11') in /config/www/gallery/include/dblayer/functions_mysqli.inc.php on line 856



Warning:  [mysql error 1064] You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ')' at line 4

SELECT id
  FROM piwigo_images
  WHERE storage_category_id IN (
)
; in /config/www/gallery/include/dblayer/functions_mysqli.inc.php on line 856


Fatal error: Uncaught Error: Call to a member function fetch_assoc() on bool in /config/www/gallery/include/dblayer/functions_mysqli.inc.php:917 Stack trace: #0 /config/www/gallery/admin/include/functions.php(71): query2array('\nSELECT id\n FR...', NULL, 'id') #1 /config/www/gallery/admin/site_update.php(445): delete_categories(Array) #2 /config/www/gallery/admin.php(309): include('/config/www/gal...') #3 {main} thrown in /config/www/gallery/include/dblayer/functions_mysqli.inc.php on line 917