As a Metada Manager user (and Crossref support team member), I'd like a mechanism within the tool to prevent me from submitting too much content at once, thus throwing an error for my submission
As a part of investigating this issue - metadata_manager#22 (closed) - we learned that the type errors reported by the users in that issue, and likely other users, was due to the volume of articles that were being submitted for deposit.
I did some testing and have found that for these accounts any submission with about 30 or fewer articles was processed successfully and anything with more than that amount generated the "type error" message (irrespective of the individual articles included in the submissions).
Jason has informed me that there is a 10M limit for submissions. The Metadata Manager submission process UIUX should be reviewed and updated to reflect this fact, so that users can no longer unknowingly exceed that limit and generate an error message.