The issue was that you had many students with many emails to send and that took some time while auto-assigning & auto sending. That the process times out and caused the process to be incomplete.
When you publish an assessment/practice test, we auto-assign all students on the groups that you deemed access to this assessment, and as a result of that we also send out notifications to these students in their SwiftAssess account as well as to their email addresses.
We have now applied a quick update to disable sending out emails as part of the notifications when assigning students. In addition to that, we have also disabled the auto-assign of students on “Practice Test” assessments since it can be done at any point of time to many number of students repetitively.