When the minecraft launcher is closed, a process remains open. This appears to prevent the launcher from reopening, as it does not yet think it is closed. Ending the task in task manager enables the launcher to be reopened.
Steps to recreate:
1) Open launcher
2) Close launcher
3) Open task Manager
4) Observe process still running.
Thank you for your report!
We're tracking this issue in MCL-9488, so this ticket is being resolved and linked as a duplicate.
Issue is (currently) marked as "works as indented", the workaround for this problem is to go to Task Manager and use 'Force quit' the process that has been stuck.
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