Thanks to this forum post I was able to find the error. The complete correct configurations can be found in the post
changes on apache at server 1 (reverse Proxy)
Old version
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ProxyPass / https://Server2.IP.*.*/nextcloud
ProxyPassReverse / https://Server2.IP.*.*/nextcloud
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new version
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ProxyPass / https://Server2.IP.*.*/
ProxyPassReverse / https://Server2.IP.*.*/
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changes on apache at server 2.
Old version
<IfModule mod_ssl.c>
<VirtualHost *:443>
ServerName *.*.*.eu
ServerAlias Server1.IP.*.*
DocumentRoot /var/www/
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new version
<IfModule mod_ssl.c>
<VirtualHost *:443>
ServerName *.*.*.eu
ServerAlias Server1.IP.*.*
DocumentRoot /var/www/nextcloud/
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changes in config.php
old
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'overwrite.cli.url' => 'https://localhost/',
'htaccess.RewriteBase' => '/nextcloud',
'overwritewebroot' => '/',
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'overwrite.cli.url' => 'https://localhost/',
'htaccess.RewriteBase' => '/',
'overwritewebroot' => '/',
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