After being updated to v1903, I find that I cannot copy files to my QNAP server over thunderbolt. I can still copy to it over the network, and can read from it over thunderbolt, just not write.
It seems it just sits on the 'calculating' dialog box forever, and ultimately tells me that the directory is no longer available. Again, I can read from it, and I can write to it via network.
I have also tried teracopy to avoid the calculating phase, but it produces similar results, so I am starting to believe something broke in 1903.