Hello @Charles Hui,
In ODPS Developers Guide, section Performance, we find test stats per message type. The performance should differ reasonably, per specific environment, these should be considered a ballpark, a rough estimate of the expected load handling.
Invalid username does not constitute an invalid subscriptionTest request. Therefore, I would expect the total number of requests per type to be the deciding factor on the acceptable load, not of the requests on this username.
I would try to verify the overall ODPS load prior to it stopping to respond, and per specific request load. Did the issue clear out, consequently, were the next requests successful? Would also try to check if the machine it is running on was stable and good on system resources at the time.