I have a form with 2 attachment fields with different IDs and different XML paths and I'm the setting on both fields for 'Max attachment in queue' is eqaul to 1 as I only want 1 attachment of each type but after I have made the setting change I'm only allowed to add 1 attachment in total when I publish the form into TM .. in Maestro its ok.
Figured it out... the name of the attachment in both attachment fields was the same and so this was causing the problem with the 'Max attachment in queue' being used for both attachment fields
That's right, the in-form check applies maxima per component instance, but the TM service doesn't know about component instances and enforces maxima by attachment name, hence the server error you saw.
Have you checked the attachment rules settings for the form in TM?
Access the Attachment Rules tab from the form Dashboard.