Trying to upload multiple files leads to network error

Hi,
I recently upgraded both ubuntu from 16.04 to 19.04 and seafile pro from 5.1.8 to 7.0.8

Since then I can only upload one single file (browser as well as ios-client). If I try to upload multiple files I receive a network error. After receiving this error I cannot upload any file at all. Config wasn’t changed since upgrading.

ccnet.conf:

[General]
USER_NAME = seafile
ID = xxx
NAME = seafile
SERVICE_URL = http://seafile.mydomain

[Client]
PORT = 13419

[Database]
ENGINE = mysql
HOST = 127.0.0.1
PORT = 3306
USER = username
PASSWD = password
DB = seafile_ccnet_db
CONNECTION_CHARSET = utf8

seahub_settings.py:

SECRET_KEY = "kindareallysecret"

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': 'seafile_seahub_db',
        'USER': 'username',
        'PASSWORD': 'password',
        'HOST': '127.0.0.1',
        'PORT': '3306'
    }
}
FILE_SERVER_ROOT = 'http://seafile.mydomain/seafhttp'
HTTP_SERVER_ROOT = 'http://seafile.mydomain/seafhttp'

apache2 virtual host:

<VirtualHost seafile.mydomain:80>
    ServerName seafile.mydomain
    # Use "DocumentRoot /var/www/html" for Centos/Fedora
    # Use "DocumentRoot /var/www" for Ubuntu/Debian
    DocumentRoot /var/www
    Alias /media  /home/username/seafile/seafile-server-latest/seahub/media

    RewriteEngine On

    <Location /media>
        Require all granted
    </Location>

    #
    # seafile fileserver
    #
    ProxyPass /seafhttp http://127.0.0.1:8082
    ProxyPassReverse /seafhttp http://127.0.0.1:8082
    RewriteRule ^/seafhttp - [QSA,L]

    #
    # seahub
    #
    SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1
    ProxyPreserveHost On
    ProxyPass / http://127.0.0.1:8000/
    ProxyPassReverse / http://127.0.0.1:8000/

</VirtualHost>

apache2 virtual host ssl:

<IfModule mod_ssl.c>
<VirtualHost seafile.mydomain:443>
    ServerName seafile.mydomain
    # Use "DocumentRoot /var/www/html" for Centos/Fedora
    # Use "DocumentRoot /var/www" for Ubuntu/Debian
    DocumentRoot /var/www
    Alias /media  /home/usernamew/seafile/seafile-server-latest/seahub/media

    RewriteEngine On

    <Location /media>
        Require all granted
    </Location>

    #
    # seafile fileserver
    #
    ProxyPass /seafhttp http://127.0.0.1:8082
    ProxyPassReverse /seafhttp http://127.0.0.1:8082
    RewriteRule ^/seafhttp - [QSA,L]

    #
    # seahub
    #
    SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1
    ProxyPreserveHost On
    ProxyPass / http://127.0.0.1:8000/
    ProxyPassReverse / http://127.0.0.1:8000/

SSLCertificateFile /etc/letsencrypt/live/mydomain/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/mydomain/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf
</VirtualHost>
</IfModule>

Maybe somebody has an idea here.

Kind regards