Calling API using login/password of user with ACL configured results in the base-64 encoded message:
System Error Server _amt_service_ is not available
Meanwhile in the logs it can be seen that AHRC fails with the dump.
Currently ACL is not supported for PBA xml-rpc API, there is a request with id #PBA-59609 ("Support ACL in PBA XML-RPC API") to implement it.
Do not use ACL for users that use API.