HTTP ERROR 500 - after plugin install

Hi,

I just tried to add a plugin > https://wordpress.org/plugins/insert-post-ads/

The instructions were pretty easy (see here > http://www.wpbeginner.com/wp-tutorials/how-to-insert-ads-within-your-post-content-in-wordpress/)

No I can't access the site. Getting "meine-ec-karte.de is currently unable to handle this request.
HTTP ERROR 500"

Site: http://meine-ec-karte.de/

I went through the following troubleshooting sequence:
1. Renamed the plugins folder to plugins-old:
I still cannot get into the Dashboard. Instead I get a new error > “W3 Total Cache Error: some files appear to be missing or out of place. Please re-install plugin or remove /home/kevinmoseri/public_html/wp-content/advanced-cache.php.”

2. I checked the memory limit on the site – There was no php.ini file
a. So I created a php.ini and added a line > memory=128MB …
That didn’t work
b. I added this line to the wp-config.php > define('WP_MEMORY_LIMIT', '64M');
That didn’t work as well

3. .htaccess -
Nothing out of the ordinary:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# BEGIN W3TC Browser Cache
<IfModule mod_deflate.c>
    <IfModule mod_filter.c>
        AddOutputFilterByType DEFLATE text/css text/x-component application/x-javascript application/javascript text/javascript text/x-js text/html text/richtext image/svg+xml text/plain text/xsd text/xsl text/xml image/bmp application/java application/msword application/vnd.ms-fontobject application/x-msdownload image/x-icon image/webp application/json application/vnd.ms-access application/vnd.ms-project application/x-font-otf application/vnd.ms-opentype application/vnd.oasis.opendocument.database application/vnd.oasis.opendocument.chart application/vnd.oasis.opendocument.formula application/vnd.oasis.opendocument.graphics application/vnd.oasis.opendocument.presentation application/vnd.oasis.opendocument.spreadsheet application/vnd.oasis.opendocument.text audio/ogg application/pdf application/vnd.ms-powerpoint image/svg+xml application/x-shockwave-flash image/tiff application/x-font-ttf application/vnd.ms-opentype audio/wav application/vnd.ms-write application/font-woff application/font-woff2 application/vnd.ms-excel
    <IfModule mod_mime.c>
        # DEFLATE by extension
        AddOutputFilter DEFLATE js css htm html xml
    </IfModule>
    </IfModule>
</IfModule>
<FilesMatch "\.(html|htm|rtf|rtx|svg|txt|xsd|xsl|xml|HTML|HTM|RTF|RTX|SVG|TXT|XSD|XSL|XML)$">
    <IfModule mod_headers.c>
        Header append Vary User-Agent env=!dont-vary
    </IfModule>
</FilesMatch>
<FilesMatch "\.(bmp|class|doc|docx|eot|exe|ico|webp|json|mdb|mpp|otf|_otf|odb|odc|odf|odg|odp|ods|odt|ogg|pdf|pot|pps|ppt|pptx|svg|svgz|swf|tif|tiff|ttf|ttc|_ttf|wav|wri|woff|woff2|xla|xls|xlsx|xlt|xlw|BMP|CLASS|DOC|DOCX|EOT|EXE|ICO|WEBP|JSON|MDB|MPP|OTF|_OTF|ODB|ODC|ODF|ODG|ODP|ODS|ODT|OGG|PDF|POT|PPS|PPT|PPTX|SVG|SVGZ|SWF|TIF|TIFF|TTF|TTC|_TTF|WAV|WRI|WOFF|WOFF2|XLA|XLS|XLSX|XLT|XLW)$">
    <IfModule mod_headers.c>
         Header unset Last-Modified
    </IfModule>
</FilesMatch>
# END W3TC Browser Cache
# BEGIN W3TC Page Cache core
<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /
    RewriteCond %{HTTPS} =on
    RewriteRule .* - [E=W3TC_SSL:_ssl]
    RewriteCond %{SERVER_PORT} =443
    RewriteRule .* - [E=W3TC_SSL:_ssl]
    RewriteCond %{HTTP:Accept-Encoding} gzip
    RewriteRule .* - [E=W3TC_ENC:_gzip]
    RewriteCond %{HTTP_COOKIE} w3tc_preview [NC]
    RewriteRule .* - [E=W3TC_PREVIEW:_preview]
    RewriteCond %{REQUEST_METHOD} !=POST
    RewriteCond %{QUERY_STRING} =""
    RewriteCond %{REQUEST_URI} \/$
    RewriteCond %{HTTP_COOKIE} !(comment_author|wp\-postpass|w3tc_logged_out|wordpress_logged_in|wptouch_switch_toggle) [NC]
    RewriteCond "%{DOCUMENT_ROOT}/wp-content/cache/page_enhanced/%{HTTP_HOST}/%{REQUEST_URI}/_index%{ENV:W3TC_SSL}%{ENV:W3TC_PREVIEW}.html%{ENV:W3TC_ENC}" -f
    RewriteRule .* "/wp-content/cache/page_enhanced/%{HTTP_HOST}/%{REQUEST_URI}/_index%{ENV:W3TC_SSL}%{ENV:W3TC_PREVIEW}.html%{ENV:W3TC_ENC}" [L]
</IfModule>
# END W3TC Page Cache core
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

# Browser Caching
<IfModule mod_expires.c>
ExpiresActive On
ExpiresByType text/css "access plus 2 weeks"
ExpiresByType text/javascript "access plus 1 month"
ExpiresByType text/html "access plus 2 months"
ExpiresByType application/javascript "access plus 1 month"
ExpiresByType application/x-javascript "access plus 1 month"
ExpiresByType application/xhtml-xml "access plus 1 month"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType image/png "access plus 4 months"
ExpiresByType image/x-icon "access plus 1 month"
</IfModule>
# END Caching