In the .htaccess file I changed the following line:
RewriteRule .* index.php [L]
RewriteRule ^(.*)$ /index.php?q=$1 [L,QSA]
to get it to run.
(to the .htaccess I also added the line
AddType x-mapp-php6 .php
but, once it worked, I didn’t bother testing if this line actually contributed to the fix)
All credit to info from a search result to my googled query “1&1 internal server error”, found at https://www.drupal.org/node/232773