I managed to find a fix for this in another thread. Somehow an iPhone got enrolled into Jamf without a serial number, and that managed to break VPP for the entire instance. Once I deleted the problem device, VPP started assigning licenses again within 10 minutes.

Thanks for sharing. This solved a problem for us too.

In our case I know for sure that the problem device had a serial number a couple weeks earlier since I had exported all devices and found that file. It had a serial then, but it was missing a serial number now.
Removing the device instantly solved the same issue you described.

Exact same problem here so I searched inventory for a device with no serial and yep there it was a device with no serial.  I deleted it and would you guess it, VPP started assigning again properly.

It has happened to me again. I have a support case in with Jamf but now I'm waiting for it to happen again so they can look at the device to troubleshoot.
If it happens to you it may be helpful to contact them and see if they can help before removing the device from JSS.

Referencing this may help catch them up to speed and let them know it's a recurring issue now.

I actually had a case opened with them and found this while waiting for them... yeah we are going to keep an eye on the device

I referenced this thread hoping to show it's not a single fluke issue.

For us it wasn't the same device - just another random device when it happened the second time.

Not sure if it impacts it, but both iPads did report 100% used storage capacity when they lost their serial number in JSS.

This just happened to one of our student iPads today, the second time this year so far. Before I deleted it, I also noted that it reported 100% used space and 1% battery level and that it updated its inventory today. This iPad was purchased through Apple in 2016 so I'm not sure what caused this now. I did a search on Used Space of 100% and found that we have 2 other iPads that have 100% used space but they still have their serial numbers. These 2 have not checked in for over a year or more. The freshly out-of-the-box iPads that I enrolled this morning are now installing apps!  ¯\\_(ツ)_/¯ 

Nearly identical findings to what I've made.

Thanks for sharing!

I'm hoping it gets on JAMF's radar.

I was troubleshooting a VPP issue all morning and I finally found this thread.  

I searched all mobile devices for anything without a serial number and sure enough there was one.  This wasn't a recent enrollment either.  I did an inventory update and blank push on the device and it suddenly had a serial number again.  Immediately the app we were having a VPP issue with started deploying.

very strange.

JAMF support suggested that the updating inventory would help.
Glad to hear it did!

It did not for me one time and the other time I deleted the object from JSS before trying. Hasn't happened to me again since to test.

I have been fighting this since Friday morning, and opened a support call with Jamf this morning. I did find an enrolled iPad that had no serial number in Jamf....weird. Once deleted, I am now able to push apps out to devices. Thanks @kj2019 !

Glad to hear that this helped you too. Jamf suggested that I make a Smart group for "No Serial Number" with the criteria "Serial Number" - Operator "is" - Value is left blank. This checks daily and so I am able to easily see when a device has lost its serial number - for whatever reason. 

@belldesean The Smart Group is a great idea. I went ahead and created one after reading this. Happy Monday to  you! I also added it to my Jamf dashboard.

It's a great workaround, but why can't the root issue be resolved? I'm looking at you, JAMF. :)

Big thanks for figuring this issue out. Just saw this issue for the first time today and sure enough, there was an iPad with no serial number. Deleted it and the apps started deploying again.

You're welcome! It was definitely a frustrating couple of days before the "no serial number" iPad was discovered. 

Just happened to me as well.  The iPad with no serial number was at 100%, but had been in the system for 2 years prior to this w/o incident.  Deleted that sucker, and apps started flowing again.

Yep! I'm glad it was an easy fix but it was definitely a showstopper at first! Hopefully, Jamf is working on the root cause of this as we type!

Jamf is working on it but doesn't have a lot of data to go off of it seems like.

If it happens regularly it may be helpful to open a support case with JAMF so they can look for similarities in our cases.

Also, in many cases, issuing an update inventory command fixes it without having to delete the device from JSS.

So glad you posted this fix as the errors in the logs didn't give any great info about it being a device with no serial number causing the issue. Will definitely be creating a dashboard item or notification that searches for this situation in the future. Thanks!

Yes, it can definitely be a showstopper!

I got hit with this as well. Thanks for the fix, my summer school teachers are much happier now.

I have been trying the update inventory command repeatedly but it does not seem to be helping. I have also tried re-enrolling. Are there any other solutions?

Have you tried deleting the offending device that has no serial number?

yes i have deleted it from JAMF and re-enrolled it from the device using account drive enrollment.