421 too many connections from this IP error mainly occurs when you open a new FTP connection.
When BackupSheep downloads your files from the server then; by default, we download 3 files at the same time. This will consume 5-6 connections during the backup run.
If you are running backing up multiple nodes from the same server at the same time then BackupSheep connections to your server will be high and it may end up reaching your server connection limit.
If you’re using shared hosting, this error may also happen because of hosting provider limits simultaneous FTP connections to prevent server overload.
How to fix this error?
There are several ways to avoid this error. One of them is to change the number of Parallel Downloads in your website node settings.
Goto your node detail page. Then click on Modify button in the actions box.
Default Parallel Downloads are set to 3 and BackupSheep will automatically set to 1 if we see this error. You can change this number based on your server limitation.
Setting Parallel Downloads to a high number can cause a high CPU load on a small server. Use caution, and don't run all your node backups from the same server simultaneously.
Configure schedules for nodes on the same server in such a way they don't overlap. This way your server won't see too many connections coming from BackupSheep and won't trigger the block.