Hi @trampas
The fatal alerts you are getting are because the client couldn’t handle the certificate you sent.
Could you specify what the difference between conditions of the runs? Is it same client all the time?
Have you checked memory leak on your side?
Have you compared your certificate message between success and failed attempts?
Since you have some limitations on memory, have you considered reducing the value of MBEDTLS_SSL_MAX_CONTENT_LEN ?
As for time enabled on your device, this could cause a problem, but only if your server requires to verify client certificates.
Regards,
Mbed TLS Support
Ron