It seems that "/media/NAS_Disk_2/The Archives/External Hard Drive/NAS_Library/Drive_1/Audio Dramas" is part of the right-side (destination-side) of your Mirror sync, and that the user credential with which you connect to /media/NAS_Disk_2/ does not have write access to said directory.
You have to organize that outside FFS.
• Find out what user credentials your Linux system uses when connecting to your NAS, and in your NAS provide write access to the relevant directories for those user credentials
OR
• On your NAS, find out which NAS user(s) have write access to the relevant directories, and on your Linux system mount your /media/NAS_Disk_2 resource using any of the user credentials that have write access, e.g. as described here.
So, it turns out I *didn't* have access to that particular folder even in Dolphin. I don't know how it got to like this, but that one specific folder had different permissions than even the other ones in the same folder.