The issue is that Horizon Client 4.10.0 dropped support for TLS 1.0 which all versions prior to 6.2.1 used. The last Horizon Client to officially support 5.3.x was 4.4 which was released on 16 March 2017. There is a workaround available for iOS but I haven't seen one for Mac. Jon Holloway or kdenike is there a workaround for Mac or Windows that you can share?
Horizon View 5.x went end of general support on 2016/09/14, end of technical guidance on 2018/09/14 and end of availability on 2013/09/30 for View Enterprise. I would urgently advise upgrading to 7.x given that Horizon View 6.x goes end of general support in less than 6 months (2019/06/19).