Manual backup with user info

Hi,

For some mysterious reason, we can't perform a manual backup with user info.

As admin, I click "backup", check the "Include enrolled users" checkbox, click "Next", and get a red "X" under "User Data" (for all items):

I'm pretty sure it's an overlooked setting, because it happened on both 2.6 and 2.8, but does not happen on a fresh installation of 2.9. I've been trying to figure it out for weeks, and I'm about to give up. 

Any suggestions? 

Thanks, 

Inbar.