I’m having the same problem.
I renewed my Premium subscription, attempted to refresh the connection (didn’t work), and then I de-installed the plugin, downloaded and reinstalled, but that didn’t work either.
I’m still seeing “Your paid access to UpdraftPlus updates for this site has expired. You will no longer receive updates to UpdraftPlus” and “UpdraftPlus warning: This is a WordPress multi-site (a.k.a. network) installation. WordPress Multisite is supported, with extra features, by UpdraftPlus Premium, or the Multisite add-on. [. . .].”
Please advise.