This caution sign means that the Host's version is outdated (i.e. with respect to the Viewer version, not in absolute figures). The information about Host version on the Viewer side is updated the next time you log in on the remote Host from that Viewer. In earlier program versions when you updated the Host and haven't logged in on it yet there may still be a caution sign even if the Host status is Online.
In the future we'll make the program update Hosts' "update statuses" immediately during the online status without the user having to log in on the Hosts. Meanwhile, in the latest RU update (18.104.22.168) we partially resolved this issue so when you run the Simple update process the caution sign disappears right away without waiting for update status confirmation by logging in on the Host.