I have given rsync a try, suggested by tk as a much better option than ftp.
Feb 21 17:31:22 raspberrypi weewx[3741] DEBUG weewx.reportengine: Running report 'RSYNC'
Feb 21 17:31:22 raspberrypi weewx[3741] DEBUG weewx.reportengine: Found configuration file /home/jjk/weewx/skins/Rsync/skin.conf for report 'RSYNC'
Feb 21 17:31:22 raspberrypi weewx[3741] DEBUG weewx.reportengine: Cannot read localization file /home/jjk/weewx/skins/Rsync/lang/en.conf for report 'RSYNC': Config file not found: "/home/jjk/weewx/skins/Rsync/lang/en.conf".
Feb 21 17:31:22 raspberrypi weewx[3741] DEBUG weewx.reportengine: **** Using defaults instead.
Feb 21 17:31:22 raspberrypi weewx[3741] DEBUG weeutil.rsyncupload: rsyncupload: cmd: [['rsync', '--archive', '--stats', '-e', 'ssh', '/home/jjk/weewx/public_html/', 'jens...@linux83.unoeuro.com:/public_html/hytte']]
Feb 21 17:31:22 raspberrypi weewx[3741] ERROR weeutil.rsyncupload: rsync reported errors. Original command: ['rsync', '--archive', '--stats', '-e', 'ssh', '/home/jjk/weewx/public_html/', 'jens...@linux83.unoeuro.com:/public_html/hytte']
Feb 21 17:31:22 raspberrypi weewx[3741] ERROR weeutil.rsyncupload: **** Permission denied, please try again.
Feb 21 17:31:22 raspberrypi weewx[3741] ERROR weeutil.rsyncupload: **** Permission denied, please try again.
Feb 21 17:31:22 raspberrypi weewx[3741] ERROR weeutil.rsyncupload: ****
jens...@linux83.unoeuro.com: Permission denied (password).
Feb 21 17:31:22 raspberrypi weewx[3741] ERROR weeutil.rsyncupload: **** rsync: connection unexpectedly closed (0 bytes received so far) [sender]
Feb 21 17:31:22 raspberrypi weewx[3741] ERROR weeutil.rsyncupload: **** rsync error: unexplained error (code 255) at io.c(235) [sender=3.1.3]
Feb 21 17:31:23 raspberrypi weewx[3741] DEBUG weewx.restx: StationRegistry: Failed upload attempt 3: b'FAIL: Server is undergoing maintenance\n'
Feb 21 17:31:23 raspberrypi weewx[3741] ERROR weewx.restx: StationRegistry: Failed to publish record 2023-02-21 17:30:00 CET (1676997000): Failed upload after 3 tries
Apparently, something is wrong with my permissions - I have the private and public keys in the right place for passwordless ssh - but nevertheless get: Permission denied (password).