I also have this error with 4.26 after upgrading from 4.25. I’ve not yet figured out what the issue is but i’ll be working on it and will let you know if I manage to find anything. Is this still an issue for you?
Thanks
EDIT
I found that i have files visible in Windows Explorer in these directories that are not part of the UE4 Asset browser, i assume this is the problem. I am going to try deleting all the explorer files that i don’t need and ensure that none of my material references break, then will package again. I hope this will fix it.
It could be a 4.25->4.26 upgrade issue.