Mbed TLS Server MBEDTLS_SSL_VERIFY_REQUIRED handshake fails

Basic description:
Mbed tls (version mbed TLS 2.28.5) is run as a server.
The authmode used here is MBEDTLS_SSL_VERIFY_NONE.
Everything works fine so far.

Now a client is to authenticate itself using a certificate.
For this the authmode was changed to MBEDTLS_SSL_VERIFY_REQUIRED.
I have created a client certificate and signed it with the same PKI.
The client certificate was generated with openssl:
openssl ecparam -genkey -name prime256v1 | openssl ec -out fixtureKeyTest.key -aes128
openssl req -new -sha256 -key fixtureKeyTest.key -out testCsr.csr -subj “/C=>> CountryName<</O=>Organization<</OU=>> OrganizationalUnit <</CN=>> CommonName <<”

When I try to establish a communication with the server using postman,
the handshake in the MBEDTLS_SSL_CERTIFICATE_VERIFY step fails with the error mbedtls_pk_verify() returned -19968 (-0x4e00).

I would be very grateful for any approaches to further troubleshooting or fixing =)