Since yesterday evening, neither LiquidBounce nor LiquidLauncher could sign in with Microsoft, and every attempt failed with Invalid app registration. Mojang removed us from the allow list of third-party applications permitted to talk to the Minecraft API.
LiquidLauncher v0.6.0 and LiquidBounce v0.40.0 for Minecraft 26.2 fix this, so please update. The old launcher cannot sign in at all anymore, and if it does not offer you the update by itself, install it manually from the download page. Your Microsoft account has to be added again in both the launcher and the client, because old sessions cannot be carried over. If you import cookie alts, use Device Code instead of the browser login, which has been removed.
Every launcher signs you into Minecraft through an application registered with Microsoft, and Mojang keeps an allow list of them. Ours had been on it for years and was taken off without warning, even though their policy states that "existing applications, such as launchers and websites, will continue to have access without interruption". We contacted Mojang, but it no longer matters what they answer. Both projects switched to minecraft-auth-rs and minecraft-auth-java, both based on RaphiMC/MinecraftAuth, and now sign in the same way the official Minecraft launcher does, which cannot be blocked without blocking the official launcher as well. The fixes are #461 and #9081.
One thing improved along the way. LiquidLauncher now opens a real Microsoft login window instead of only offering a device code, and LiquidBounce lets you choose between Web View, Device Code and Credentials.
We still think this was the wrong call. Logging in through your own browser was safer, because the launcher never came anywhere near the form you typed your password into. Now you have to trust that we do not read from the window we open for you, which is a bigger ask even with our source code being open.