Leaving this here for now - the issue is probably not on the side of solid-nextcloud, but it could be useful for others:
Workaround for /session (note: NOT /apps/solid/session , but plain /session).
add php file 404.php in /var/www/html/:
<?php
http_response_code(404);
add a redirect rule for ^session* to 404.php
prevent the nextcloud redirect from happening by making an exception in the rules that head to index.php
in the first block:
RewriteRule ^session* 404.php [L]
in the second block:
RewriteCond %{REQUEST_FILENAME} !/404.php