I managed to find a solution, for those interested. The problem was that I tried to perform a prepared statement with a lot of parameters, which surpassed the build in limit of 2^15-1, so my solution was to chunk up the queries within the transaction:

(jdbc/with-db-transaction [db @datasource]
  (doseq [chunk (partition-all 1000 (get-in req [:body :items]))]
    (insert-all-items db chunk)))