I noticed the same behavior with tumbler posting:
High success rate with 1 thread and success rate goes down with each more thread being used. each account has its own private proxy.
It seems that every action happens nearly at the same time within each thread and/or ZP has hiccups when using...
I have the same issue:
Creating hotmail accounts, 20 proxies, each used once then deleted from list. cannot create more than 4 in a row. then error 450: try again the next day.
re-running the template (=closing browser) does not help.
:(