There is error in log:
530 Sorry, no more connections for your ip
option batch continue option confirm off open ftp://user:password@server -timeout=120 synchronize remote -delete -filemask="| $RECYCLE.BIN/; System Volume Information/; *.bak; *.tmp" E:\ /E/
Answer from ftp support:
in this case, you have simply used all the available connections for your client.
There is a limit of 10 concurrent connections. This is a hard limit, which we wil not raise.
The limit counts separately for WebDAV, SAMBA/CIFS, and FTP/SCP/rsync.
The limit is also separately for sub-accounts. So if you have multiple entities, accessing the storage box, using sub-accounts is highly recommended.
If you do not have any open connections any more, please check, if your software terminates the connections properly, so they are freed on our side as well.