hi @ncw thank you very much. I will have a look if switching to SFTP is feasable. In the past I thought that especially the limitations mentioned concerning Synology NAS systems are good reasons to avoid it. Another user asked what I also wanted to ask a while ago: "On some SFTP servers (e.g. Synology) the paths are different for SSH and SFTP so the hashes can't be calculated properly" Maybe an update there will help me to figure out if SFTP is a good choice.
I also filed an issue here (because the FTP server on Synology NAS is a SmbFTPD): twbsd.org
I am also checking if all this is also related with issues in goftp like this one here: github dot com/jlaffaye/ftp/issues/249
There is never exactly the same issue but they seem to be nearly the same 