destination? Version info? Libcurl info?
Try process manager and provide a log. Might not help if the issue is on the network location, but then you'd know that info at least
I have the same error: rename file failed with big files. The remote filesystem is special because it versionize files, and the SFTP server mapped on it seems to handle temporary files on a different file system to avoid the "rename" versionning. That's why the rename command is supported by the SFTP server but leads to an error due to an impossible rename from a file systems to another file system.
So the solution I use in winSCP is to switch the flag -resumesupport=off
I would like to disable the resuming in freefilesync the same way as winSCP, but I don't know how to do that. Is it possible?