If you got ERR_SSL_PROTOCOL_ERROR on web page running on IBM i. You can find something like this in HTTP server log
[Fri May 03 12:31:22.022600 2024] [ibm_ssl:error] [pid 227529:tid 0000002E] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 12:31:22.022592 2024] [ibm_ssl:error] [pid 227529:tid 0000002C] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 12:31:22.024928 2024] [ibm_ssl:error] [pid 227529:tid 0000002F] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 12:31:22.026448 2024] [ibm_ssl:error] [pid 227529:tid 0000002E] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 12:31:22.026464 2024] [ibm_ssl:error] [pid 227529:tid 0000002C] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 12:31:22.026464 2024] [ibm_ssl:error] [pid 227529:tid 0000002F] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 12:31:22.028608 2024] [ibm_ssl:error] [pid 227529:tid 0000002B] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 12:31:22.028632 2024] [ibm_ssl:error] [pid 227529:tid 0000002B] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 14:10:19.403256 2024] [ibm_ssl:error] [pid 227529:tid 0000002E] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 14:10:19.403632 2024] [ibm_ssl:error] [pid 227529:tid 0000002E] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 14:10:19.403256 2024] [ibm_ssl:error] [pid 227529:tid 0000002B] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 14:10:19.404128 2024] [ibm_ssl:error] [pid 227529:tid 0000002B] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 14:10:19.409992 2024] [ibm_ssl:error] [pid 227529:tid 0000002F] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 14:10:19.410008 2024] [ibm_ssl:error] [pid 227529:tid 0000002F] ZSRV_MSG0281: SSL: 10.53.122.32.
[Fri May 03 14:10:19.410200 2024] [ibm_ssl:error] [pid 227529:tid 0000002E] ZSRV_MSG0214: Improperly formated certificate or certificate not valid, error = 415.
[Fri May 03 14:10:19.410208 2024] [ibm_ssl:error] [pid 227529:tid 0000002E] ZSRV_MSG0281: SSL: 10.53.122.32.The cause may be the use of a certificate with an ECDSA key of 384 or 521 bits. Edge and Chrome browsers may have a problem with such a certificate.
You can try to create new server certificate with ECDSA 256 bit key or RSA keys.
Or Firefox browser should display page correctly.