Exchange 2016 Decommission

Hi all,

I am trying to decom my 2016 environment and cannot disable (Disable-Mailbox) the final 16 mailboxes (out of thousands). Am getting the error 

Active directory response: 00002098: SecErr: DSID-03150A45, problem 4003 (INSUFF_ACCESS_RIGHTS), data 0.

I have been troubleshooting this for HOURS now. It's not enabling inheritance on the user perms (every article suggests that). The perm exchange trusted subsystem isn't there. The users do not have any admin groups. It's nothing that I can find in the first 6 pages of googling. Sadly, of course, there is no support anymore so can't even raise a ticket.

Can't uninstall Exchange with mailboxes left :(

Any idea's are more than welcome, please.

Thank you.