Uart framing error on Stm32l496ag mcu (some transmit bytes are corrupted) . What is the reason ?

[OGOKC]

I send 1 byte data('A') per second via a UART4 interface. Below is a screenshot of the data received via Realterm. As you can see sometimes a corrupted character appears. My codes is below. What could be wrong where of the code? I'd appreciate it if you could help. Thanks.

[Tesla DeLorean]

Is this a direct connection, or via BlueTooth, or RS232 level converters.

Review the signal integrity of the USART_TX pin with a scope. Check timing, slew rate, ringing, etc.

[OGOKC]

Connection is via USB. The circuit is as follows. i haven't looked at UART TX signal yet. i will look. I suspected uart clock. it looks right.

[OGOKC]

I have looked uart tx signal via logic analizör. Sometimes framin error occurs. Correct and faulty frames are given below. Task scheduler is closed. Only uart data frame is transmitted per 2.4 second for 40 minutes. I suspected uart clock. I checked , it looks right.