Java Username Prompt After Account Migration

Background

I migrated my Mojang account to Microsoft in 2022. For years everything worked. After a Windows update in late 2025, the launcher started asking me to "enter a Java username" every session: even though I'd set one in 2015.

What Didn't Work

• Re-entering the username (it accepted it but asked again next launch)
• Reinstalling the launcher
• Changing Microsoft account password

What Worked

Closed launcher. Went to %appdata%\.minecraft and deleted only launcher_profiles.json. Did NOT delete saves or mods. Restarted launcher, signed in, set username once. It stuck.

The file apparently held stale Mojang-era profile references that conflicted with Microsoft authentication after the update.

Guide: Java username prompt fix.