Internal server error at migration from Joomla to WordPress

Hey guys,
I would like to migrate our Joomla site based on the latest version 3.9.20 to the latest WordPress version 5.4.2. The WordPress installation is completely blank except the FG Joomla to WordPress plugin.

I added a php.ini file to the installation directory with the following content:

register_globals = off;
magic_quotes_gpc = off;
allow_url_fopen = on;
safe_mode = off;
file_uploads = on;
memory_limit = 512M;
upload_max_filesize = 64M;
post_max_size = 64M;
max_input_time = 0;

In addition to this the wp-config.php file looks like this:

...
define( 'WP_DEBUG', false );
define('WP_MEMORY_LIMIT', '512M');

/* Das war’s, Schluss mit dem Bearbeiten! Viel Spaß. */
/* That's all, stop editing! Happy publishing. */
...

I’ve checked the right configuration by an phpinfo.php file. The memory limit is shown by 512M. What can I additionaly do for a complete migration?

The webserver is based on a sharing hosting package at IONOS in Germany.

Thanks in advance,
michai

### BEGIN DEBUG INFO ###

WordPress info:
  Plugin URL: https://neu.sv-harderberg.de/wp-admin/admin.php?import=fgj2wp
  Site URL: https://neu.sv-harderberg.de
  Home URL: https://neu.sv-harderberg.de
  WP version: 5.4.2
  WP Memory limit: 512M
  Multisite: no
  Permalink structure: /%year%/%monthnum%/%day%/%postname%/
  Media in year/month folders: yes
  Active theme: Twenty Twenty 1.2
  Active plugins: 
    FG Joomla to WordPress 3.68.0

PHP info:
  PHP version: 7.4.7
  Web server info: Apache
  memory_limit: 512M
  max_execution_time: 7200
  max_input_time: -1
  post_max_size: 8M
  upload_max_filesize: 100G
  allow_url_fopen: 1
  PDO: sqlite, mysql

MySQL info:
  MySQL version: 5.7.30
  max_allowed_packet: 67108864

Plugin info:
  FG Joomla to WordPress 3.68.0
  Add-ons:
  Options:
    automatic_empty: 1
    url: https://sv-harderberg.de
    hostname: ***
    port: 3306
    database: ***
    username: ***
    password: ***
    prefix: ***
    introtext: in_content
    archived_posts: published
    skip_media: 
    featured_image: first_image
    only_featured_image: 
    remove_first_image: 
    remove_accents: 
    import_external: 
    import_duplicates: 
    force_media_import: 
    meta_keywords_in_tags: 1
    import_as_pages: 
    timeout: 10
    logger_autorefresh: 1

### END DEBUG INFO ###