@murgero Great, thank you for the suggestion! Hopefully the team will still consider this valid request, but appreciate this workaround.
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RE: OnlyOffice Document Security Token Issue
The two strings in secret was changed, but the 'Secret key' in the Nextcloud ONLYOFFICE admin panel still had the old default value.
For others in the future, please don't forget to double-check the values changed in production-linux.json is also reflected in the Nextcloud ONLYOFFICE configuration area.