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Sorry to hear you’re facing problems 
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Nextcloud version (eg, 12.0.2): 15.0.2
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The issue you are facing:
Under “Security” and “Basic Encryption Module”, there used to be two fields, where one could type in the current log-in password and the old password (encryption key), allowing a user to regain access to their files, which were encrypted with the old password. These fields have disappeared in NC 15. Some of our users got stuck in a state where, after password reset, they have no way of gaining access to their encrypted files, even though they know both their current and their old password. Any ideas?
Steps to replicate it:
- With the basic encryption module enabled and per-user encryption keys (no master key), reset a user’s password via the “forgot password” on the log-in page
- Follow the emailed link, change password and log in to account
- Under “Security” and “Basic Encryption Module”, the two fields, current log-in password and old password, are no longer there