Ok, that's clear, thanks. Would be nice though

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In the mean time I managed to reset the options by tampering with the database itself. Note that this is a hack, and probably not recommended. I happen to know my way with this stuff reasonably well and am prepared to take risks (though minimal, since I've made a backup). Needless to say, use this at your own risk

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First (optional but recommended), go to your library folder and backup
metadata.db.
Then download
SQL Database Browser, or any equivalent tool of your liking.
Use this tool to open
metadata.db in your library folder, click the
execute SQL tab and enter the following query:
DELETE FROM conversion_options; (tech: TRUNCATE TABLE is not supported in SQLite) and press
Execute query. This removes
ALL saved conversion settings and thus resets them to default for each and every book and every format. If you know your way around Calibre's database structure and SQL you can of course alter this query to affect only specific books or formats.
Lastly, save the file and close the program.
If you open Calibre the saved conversion settings should have disappeared. In case anything goes wrong just put back the backup you (should) have made and you'll be fine - though you're still stuck with the conversion settings

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Good luck.