RTX, MessageQueue, Realtime desaster

We have four identical µC systems based on STM32G081, running same, identical firmware.
They receive commands via one, single RS-485 bus.
One job is switching a GPIO pin from on to off.
UART interrupt has highest prio (0), consuming task has highest prio (realtime).
Occasionally one or more system (see LA pictures) are delayed.
Delay is always the same (@24 MHz 3.7 ms, @ 64 MHz 1.8 ms).

May someone help in what is blocking the consuming task ?

Best regards, Juergen

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