It appears a recent nginx update, 1.18.0-6ubuntu14.19 to patch CVE-2026-42533, broke Nextcloud [for me?].
MIAB uses variables to construct the FastCGI filename, which nginx passes to PHP-FPM with a corrupted path “/usr/local/lib/owncloud//cloud/ind” instead of “/usr/local/lib/owncloud/index.php”; this means your Nextcloud instance shows “File not found” coupled with people complaining their shared calendars are broken this morning 