This could be because there is a mismatch between the SSL/TLS settings between vTM and the back-end nodes - for instance, if vTM attempts a type of TLS connection which has been disabled by the server.
If you enable log!server_connection_failures and log!ssl_failures on the Virtual Server, then you should see more information about what is going on, but the best thing to do here would be to raise a support call so our team can take a closer look at the problem.
