The error message on its own does not clarify the context of that error.
Please copy the complete Java error stack everytime you seek assisatnce from anyone, not just Marketo.
To clarify, that sort of error is presented as
Caused By: java.net.SocketException: Connection reset
at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:96)
at java.net.SocketOutputStream.write(SocketOutputStream.java:136)
(long stack, supressed here as it is intended to be an example only)
The error I posted above was thrown by Oracle BEA/Weblogic running on my localhost.
That sort of error is unrelated to Marketo or SOAP API. I forced it directly through PHP initiating a call to BEA/Weblogic running on a second localhost.
The lack of details allow me to post generic suggestions only.
It may be a symptom of a thread pool exhausted by initialization or an application server having trouble procesing blocks equal or geater than 4 KB.
In the syncMultipleLeads context that could be mitigated by a significant reduction in the number of leads per API call. Try half to start troubleshooting and, if no error, try gradual increases. There is no generic guideline since the block size is a combination of number of records and number of attributes per record.