with my new Ubuntu installation I run into an error. Via Ubuntu 20.04 Software Manager I can install Arduino 1.8.13. It works and a board is selectable via the Boardmanager. However once I have install the 8266 Extension via the Librarymanager, I'm no longer able to select a Board. The BoardManager shows the Arduino and 8266 categories, however it doesn't show the sub-selection for the categories.
On Windows it works without problems.