FATAL ERROR: Connection reset by peer (could not connect to server)

It may happen that a client that has always been able to connect to Syncplify Server! without any issues, suddenly can't connect and displays a message similar to this one:

FATAL ERROR: Connection reset by peer (could not connect to server)

Depending on the client software the failure message may vary, but its meaning is always the same: the client cannot connect to the server.

When that happens, the most likely causes are:

1. the client software has violated the protocol or the permissions (for example it has tried to upload a file to a folder where uploads are forbidden) multiple times, and has been block-listed - in this case, locate your client IP address in the block-list, and remove it
2. you may have added at least 1 IP address or network to your allow-list, but now you're trying to connect from an IP address that's not allow-listed; please keep in mind that the allow-list, when used, is restrictive; learn more about the allow-list (and other lists)
   
3. if you're running an old version of Syncplify Server! (v1-v5) your server may have detected changes in your hardware, virtual hardware, or operating system, that may have led to an invalidation of the license - licenses that revert back to FREE edition cause the server to only accept a single client connection at a time - if that happens submit a ticket here (top right corner) and we'll help you reset your license
4. client and server may have been unable to negotiate a common set of encryption/MAC/key-exchange algorithms; in this case it's strongly recommended to configure the client to use the same strong algorithms available on the server, weakening the server by adding weak algorithms to it is never a good idea