External RFC logon not possible when auth/rfc_authority_check is set to 9


we have to logon via RFC to an ECC6 system where auth/rfc_authority_check is set to "9" but SAP gives 'RFCAPI_RFC_SYS_EXCEPTION:RFC_ERROR_SYSTEM_FAILURE' error. If this parameter is set to "1" then logon is ok. I checked Saphelp and Sapnotes but the documentation for this parameter is a little bit unclear. I tried to change authorizations but without success. Does anyone have experience with this topic?

Many thanks in advance for your help!