Can you get into it and then your session ends and you're logged out immediately?
What's your server environment? Web server, relevant applications (php-fpm, redis, etc)
When did this start occurring and what changes were made to the server immediately prior?
Does this happen with everyone with admin panel access or just you? If there are no other admins then someone can provide database queries to manually make someone else an admin for debug reasons.
Are you accessing multiple XF installations? I have it when browsing between test/live. Perhaps it's even when accessing any other forum with the same cookie names?
A change of IP could be a reason. But as James said, more info is required to fully understand what's going on.
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