Avoiding "too many active scans" on MySQL Cluster
"Too many active scans" error means that at a given moment there are too many queries running which are scanning the tables while trying to get answers.

Scanning means for example range scan queries (WHERE val > 20). And if I remember right scan also means fetch queries if index is not has.

SELECT * FROM tbl WHERE id=20;


Even such query counts as scan if id field has btree index.

In your case we almost all DELETE queries are running in loop with 'LIMIT 1000' or similar value.