UPDATE: with the great people at Synology we managed to locate the issue.
It appears the problem happens with the Mac (in Tahoe) tries to copy the invisible .DS_Store file to the NAS. Given the .DS_Store file is no longer made visible with the Cmd-Shift-Period keystroke, you have to use Terminal to get rid of it long enough to make the copy before the Mac rebuilds the DS_store again.
The most reliable way to do this is copy the problem folders & files to the desktop first then use terminal to delete the .DS_Store files in the folder.
Terminal commands:
Step1
cd (file path here)
*** changes path in Terminal
Step2
ls -lahr
**** shows contents of the path-folder
**** if there's a .DS_Store there then move on to Step 3
Step3
rm -f .DS_Store
**** deletes the DS_Store file
OR... you can try a general search & destroy inside the folder
find (filepathhere) -name ".DS_Store" -type f -delete
But be very careful!!!!!
Working with Synology and Apple now to find a better long-term solution.