i would lean more towards a logic error in the trigger.

the login trigger clearly rejected the login, but that might be because the login user does not have read permissions to sys.dm_exec_sessions, or the logic to write to a log might be killed due to permissions to write to the table, or goes down a logic tree of IF statements that raise the error, but don't write to your log.

show us the trigger itself, and we can offer some peer review.