TYPO3 Core

What kind of extension/action setup/workflow do you use here, where you could need more information?

From what I understand, the message is intentionally meant vague, because it's revealed to consumers and should not carry possibly security-relevant information about the internal application. I believe it should only happen when an extbase action validation fails, which should only happen with malformed requests. If you want to reveal the errors, you might want to override the errorAction in your extbase controller and return a custom error template, revealing the flashmessages. Or you could instead use model validation instead of action validation to get more granular control over what kind of errors on models get reported?

You might also be able to use a custom errorAction() to read the flashmessages and forward them to a logger. The current extbase default errorAction implementation is also intentionally very basic ("This is suitable for most actions dealing with form input") so I'm not sure if it can be enhanced easily fit extended needs. For other people, extra info on "wrong use" of bots of extbase acctions being logged might significantly spam the logging facilities, so it's not easy to make it suitable for everyone.

Maybe for Staging/Development context this could be changed to directly put the flash message contents into the htmlResponse, would that help for your scenario?