Lychee: After v3 to v4 migration: Albums are there, but photos are missing

Detailed description of the problem

After migration from v3 to v4 I see all of my five albums, but four of them are empty and one of them contains only one photo. I followed the migration instructions here.

Steps to reproduce the issue

n.a.

Screenshots n.a.

Output of the diagnostics

    Diagnostics
    -------
    Info: Latest version of PHP is 7.4
    Warning: Using 32 bit PHP, recommended upgrade to 64 bit
    Warning: Dropbox import not working. dropbox_key is empty.


    System Information
    --------------
    Lychee Version (release):   4.0.6
    DB Version:                 4.0.6
    
    composer install:           --no-dev
    APP_ENV:                    production
    APP_DEBUG:                  false
    
    System:                     Linux
    PHP Version:                7.3
    MySQL Version:              10.3.23-MariaDB-0+deb10u1
    
    Imagick:                    1
    Imagick Active:             1
    Imagick Version:            1690
    GD Version:                 2.2.5

    Config Information
    --------------
    version:                    040006
    check_for_updates:          0
    sorting_Photos_col:         takestamp
    sorting_Photos_order:       ASC
    sorting_Albums_col:         id
    sorting_Albums_order:       DESC
    imagick:                    1
    skip_duplicates:            0
    small_max_width:            0
    small_max_height:           360
    medium_max_width:           1920
    medium_max_height:          1080
    lang:                       de
    layout:                     0
    image_overlay:              0
    image_overlay_type:         exif
    default_license:            none
    compression_quality:        90
    full_photo:                 1
    delete_imported:            1
    Mod_Frame:                  1
    Mod_Frame_refresh:          30
    thumb_2x:                   1
    small_2x:                   1
    medium_2x:                  1
    landing_page_enable:        0
    landing_owner:              John Smith
    landing_title:              John Smith
    landing_subtitle:           Cats, Dogs & Humans Photography
    landing_facebook:           https://www.facebook.com/JohnSmith
    landing_flickr:             https://www.flickr.com/JohnSmith
    landing_twitter:            https://www.twitter.com/JohnSmith
    landing_instagram:          https://instagram.com/JohnSmith
    landing_youtube:            https://www.youtube.com/JohnSmith
    landing_background:         dist/cat.jpg
    site_title:                 Lychee v4
    site_copyright_enable:      1
    site_copyright_begin:       2019
    site_copyright_end:         2019
    additional_footer_text:     
    display_social_in_gallery:  0
    public_search:              0
    SL_enable:                  0
    SL_for_admin:               0
    public_recent:              0
    recent_age:                 1
    public_starred:             0
    downloadable:               0
    photos_wraparound:          1
    map_display:                0
    zip64:                      1
    map_display_public:         0
    map_provider:               Wikimedia
    force_32bit_ids:            0
    map_include_subalbums:      0
    update_check_every_days:    3
    has_exiftool:               0
    share_button_visible:       0
    import_via_symlink:         0
    has_ffmpeg:                 1
    location_decoding:          0
    location_decoding_timeout:  30
    location_show:              1
    location_show_public:       0
    rss_enable:                 0
    rss_recent_days:            7
    rss_max_items:              100
    prefer_available_xmp_metadata: 0

Browser and system

Client: Firefox 78.2.0esr on Fedora 32 Workstation Server: Debian Buster on armv7l