In most cases this error means that the connection was interrupted.
It was lost, the server disconnected or your antivirus/firewall cut the connection.
In technical terms, it means exactly what the exception’s message says: component tried to read a line (ending with CRLF), the line was expected in accordance to the protocol in use, but it hasn't been received during the specified time period.
If increasing timeout values (ClientBase.ReceiveTimeout) doesn't help, it means that the connection was dropped by the remote server or some intermediary router - you'll need to connect again.
If you are using Imap.Idle you can use lower IDLE timeout. It specifies timeout after which IDLE command is re-issued. This breaks inactivity period (which happens when no new message is delivered) and thus prevents routers from cutting the connection.