I had the same issue and found a possible solution. It seems to be an xbox live login token issue, getting a wrong one from cache (maybe you changed to beta release usage and this might caused it, this is when happened to me).
Try to disconnect your internet, start Minecraft Dungeons Launcher, wait until it complains about no internet, click on your username on the top left corner and Sign Out, then close launcher (maybe even clear out beta in Settings before). Reconnect your internet and start Launcher and login normally.
Try MCL-14290