Bugs and feature requests: GitHub Issues.
Email: dp@intistele.com
Step-by-step: the user guide. How your data is protected: security.
Nothing is pasted after I pick an item. Grant the Accessibility permission: System Settings → Privacy & Security → Accessibility → enable Keepit (if it is already listed, toggle it off and on). Until then, press ⌘V yourself — the item is on the clipboard.
I forgot the password. Use Use Recovery Phrase… on the unlock dialog, or Touch ID on the same Mac. Without either, the data cannot be recovered.
Where is my data? ~/Library/Application Support/Keepit/ (App Store edition: inside ~/Library/Containers/com.it.keep/).