This error code usually happens with imtcp. This plugin has a maximum number of listeners (server ports) that can be used. That limit is set at build time. If you experience this problem, there are two solutions: if you run under linux and do not need TLS, you can use imptcp instead, which is faster and does not have this limitation. If you need to stick with imtcp, you need to do a custom build and increase the max number of listeners #define.