MUT Device SERIALNUMBER - Failed! - 400!

What does your csv look like? Do you have the allow modifying device name checked?

My csv has a serial number in column A and the name I want to set the iPad to in column B

Where are you referring to this setting:
Do you have the allow modifying device name checked?

That aside, these are all student iPads, so they should all have the same settings.

I was guessing that these errors were related to the ability of the JSS to respond to the commands fast enough, but I was hoping to get some confirmation on that, and find a way to resolve the issue.

I have a config profile for my students that does not allow them to modify the device name. If I don't check the box to allow the modification, I get those errors. Configuration Profiles - configure restrictions - iOS and tvOS - Functionality. We have only about 1,000 ipads so when I used MUT, I did it in batches (grade level).

We don't allow students to change the device name, so that restriction is enabled.

But would that setting prevent the MDM from changing the device name?
I wouldn't think it would — it would prevent an end user from changing the name, but out shouldn't prevent an MDM command from changing the name.

Also, if that were the problem, I would have 100% failure rate, not 25%.

I think I found the problem.

I had used the MUT tool to change the name on devices a couple of times throughout the day as we get a few hundred more iPads online each hour or so.

The errors appeared to be for iPads that had already been sent the Set Device Name command, but hadn't processed it yet (still in Pending).

I sent a cancel all Pending and Failed command to all student iPads this morning, and then the MUT tool worked for 100% of the devices.