This is error related with installed plugin in your Redmine: redmine_preview_pdf
redmine logs
ActionController::UnknownFormat (ActionController::UnknownFormat):
plugins/redmine_preview_pdf/lib/redmine_preview_pdf/patches/attachments_controller_patch.rb:70:in `show_with_pdf'
We added the issue for developers:
https://github.com/HugoHasenbein/redmine_preview_pdf/issues/1
Anyway you can wait the fix or disable this plugin because it's broken Redmine API.
Step-by-step guide
- Uninstall redmine_preview_pdf
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