Ah, during our migration we did try to move thousands of files from a "Dropbox" folder to a "NextCloud" folder, and indeed the Windows client was not happy. Since it was a one-time thing, the solution was to move the files "manually" over SSH and just run the NextCloud "scan" utility to pick up the changes on disk.
I'm running NextCloud via the official Docker image, reverse proxied through nginx.
I'm running NextCloud via the official Docker image, reverse proxied through nginx.