This error is reported by the operating system when the message queue buffers are full. Although it is possible that the system message queue buffer size was set too low, the more likely cause is unread messages. A common programming error is not to clear system signals after an INPUT TIM statement for an IRIS program. This error can also be reported when UniBasic, dL4, or IQ are first started if the system doesn't have a sufficient number of message queues (MSGMNI) allocated.