A possible bug :
If you try to install an addon (from https://microsoftedge.microsoft.com/insider-addons) on an inPrivate window, you will get this error :
The problem i found is that, if you switch right after to a "normal" browsing window & try to install the same addon you will get this one :
Of course, the said addon has not been installed.
Getting around this error for now require closing all browser windows & restarting it to try again (from a normal window) to effectively install it.
Found using Version 184.108.40.206 (Official build) canary (64-bit)
Hope this help, & good luck :smiling_face_with_smiling_eyes:
EDIT: Shared this info using the "Send feedback" form, & included a link to this thread for more details.
EDIT2: Same with the latest build (Version 220.127.116.11 (Official build) canary 64-bit)