I think the developer of Beem is @holger80 and recently the @blocktrades team has been updating the library as well (here: https://gitlab.syncad.com/hive/beem ) so hopefully some of them can help or maybe you can open an issue in the repository to ask for advise.

I am no python dev, nor a blockchain dev, but I did take a look at the Beem library a bit and I would think the class UnhandledRPCError does nothing except just gives you the error Beem got from the RPC node (if I'm understanding how all this works). Why would the RPC node suddenly give an error about a missing posting key authority? Maybe the node was down for a bit or had some temporary issue. If your code was running without any problems for a month, node issues would seem like one likely culprit for the failure happening. A possible mitigation then would be to check for any node-related errors in your code and try another node if the current one gives you an error.

But why did you keep getting this error for a time? A possible explanation would be that the node had an issue for some time, though it could also be something else. Again, not a python or a blockchain dev, just speaking from experience and throwing it out there in case it may be useful.