"This page can only be opened from AlertKeeper's own menu"
Each start of AlertKeeper has its own key. Open the page from the dot in the menu bar (on a Mac) or the tray (on Windows) (Open AlertKeeper), not from a saved link or another browser.
"Reconnect mailbox" or "Mailbox not reached"
Check the internet connection; AlertKeeper tries again by itself. If Google's app password was removed or changed, make a new one and connect again in Settings → Mailbox (the mailbox guide).
"An entry an earlier copy of AlertKeeper left in your Keychain (on a Mac) or Credential Manager (on Windows)"
Copies of AlertKeeper before 0.2.9 weren't signed with a Developer ID, so each was a different program to the computer and couldn't change an entry an earlier one made. AlertKeeper makes its own new entry; to tidy up, open Keychain Access, search for “AlertKeeper” and delete its entries (on a Mac); open Credential Manager → Windows Credentials, find AlertKeeper and remove its entries (on Windows). Since 0.2.9 every version is signed the same way, so it won't happen again.
"Your Keychain (on a Mac) or Credential Manager (on Windows) is locked"
Unlock it (log in again, or in Keychain Access: File → Unlock), then try again.
"no price"
Prices are saved each evening after the market closes, while AlertKeeper is open; a stock or option shows its last price "as of" its day. With the price source set to None nothing is fetched: Settings → Prices.
A broker file AlertKeeper doesn't recognise
It helps you match the file's columns and keeps the layout for that broker; or use AlertKeeper's own template.
Something red on Health
Each card says what's wrong and what to do. Health also has Copy diagnostics: a plain file you read first, never letters, trades or account details.
Beginning clean
Replacing or deleting the app keeps your data -- in ~/Library/Application Support/AlertKeeper on a Mac, in %LOCALAPPDATA%\AlertKeeper on Windows, where the uninstaller asks whether to delete it too. To begin clean, use Settings → Start over before you uninstall.