I have the same problem, although no users can edit.
We've used draw.io for a while and created many diagrams which could previously be edited from the thumbnail. The thumbnails no longer show, and cannot be edited.
We have JIRA 7.1.7 and draw.io 126.96.36.199.
(Same on Chrome, Safari, etc)
Was this resolved for you?
Presumably JIRA Settings > Permissions (or project administration).
In the relevant permission scheme there is a section "Attachments Permissions".
All the appropriate user groups are enabled here to create attachments. I have even explicitly granted my username with no difference.
In the thumbnail position under draw.io diagrams on the issue, it sometime shows the error message "undefined: MXutiils.getAll is not a function"
Did all that, no improvement.
Attachments are on; I apply attachments to many issues with no problems. The atatched draw.io files are still present on the issues, and can be downloaded ok. But no editing can be done and thumbnails missing.
Cleared cache = same.
Uninstalled Draw.io, rolled back to the previous version = same.
Uninstalled then re-installed latest version = same.
MXutils is a java component of MXdraw, which is from the same provider as Draw.io I think. So presumably it's bundled with Draw.io?
I've reported to Draw.io yesterday but no help yet.