Shoot, I would think the next step of troubleshooting would be messing with how to install the launcher. Like doing a manual download from Minecraft’s official website, or downloading the launcher from the Microsoft Store. Another thing to check would be messing with the run with admin on your desktop on the Minecraft Launcher. Hope those help if you haven’t tried those yet. I don’t have any other ideas other than a clean wipe on your hard dive, maybe something in the background is causing this.
Not a mod, but I’m trying to help as many people as I can. To help Mojang fix this, I’ll try to replicate your issue if this doesn’t help fix your issue.
So you can’t select any skins or add your own? Try logging out of the Minecraft Launcher with your Xbox Account. And sign back in. This should refresh your skin files, if this doesn’t work, a way to get around this is going to Minecraft.net to go to your profile. Go to account, click on profile. Go to all games. Click on change skin through Minecraft Java edition. See if that works as well. If not, it’s probably just an error with your account. Let me know how it goes.
I’m trying to help out as many bugs on here as possible. Did you launch the Minecraft Launcher when your Xbox Account was already on another game? If not, then I think the launcher detects that your account is still playing it, or something else. Even if it’s not on a game or on the launcher. Quick way to check is to go to your Xbox app or Gamebar (Windows Key + G) to check your status. If it shows Minecraft Launcher while you aren’t loading it. That has to be the issue. If that’s still not the case, just let me know and I’ll try to help out as best as I can.
Quick question to help replicate this issue. Did you put in un-stackable items into your Shulker Box? Also to help out with reporting this bug. Are you on an original Switch? Like a launch model? Or Lite, OLED, or HAC-002 and onwards. Or a Switch 2? Let me know and I’ll try to replicate this issue to help Mojang find the root cause of this.
They could help track down this issue. But if you haven’t already tried installing the launcher through the Microsoft Store. Try that, or installing it manually. I’m going to assume did those already. Try going into your Xbox Games folder through your O: or whatever your computer hard drive directory is. Try launching the Minecraft Launcher manually through that instead of your shortcut on your desktop. Hope this helps.