Update: as of June 9th at 4.30AM PT / 7.30AM ET / 12.30PM BST / 1.30PM CEST / 8.30PM JST, the Firmware update is available for download again.
If you encountered the issue with the microSD Card (described below), you can get it to work by inserting the microSD Card in the console, connect to the internet, and then update the console.
Earlier this week, Nintendo released the latest Firmware update for the Nintendo Switch (Ver. 12.0.3). Unfortunately, not only did this update not bring anything overly exciting (officially, only the usual improvements to stability), it’s also causing some issues with the internet connection and the microSD Card.
After updating to Ver. 12.0.3, your Nintendo Switch may have trouble connecting to the internet. Even more problematic: if you try to use a new microSD card with your Nintendo Switch after updating to Ver. 12.0.3, the “initialization” phase (a small “update” that’s automatically downloaded before you can use the card) will fail, and you won’t be able to use the microSD Card with your Nintendo Switch.
It’s not clear what’s causing those two issues, but Nintendo are already working on a fix. We don’t know when the next Firmware update will be released, but in the mean time, the company has decided to stop the distribution of Ver. 12.0.3.
If you’ve already downloaded that Firmware update, there’s really nothing to do other than refrain from using a new microSD Card with your Nintendo Switch (there’s nothing to worry about if you’ve already been using one prior to Ver. 12.0.3).
Source: Nintendo (Twitter)