After about two months TTNmon is available again at ttnmon.meis.space. The basic functionality has been restored. It is now possible to query packet metadata as well as gateway metadata. Performance is much better than before.
The map as well as the gateway explorer are currently still unavailable. I hope to finish the implementation of these modules soon.
It is now no longer required to give TTNmon access to your devices list. Device EUIs are now connected to the authentication token. This circumvents the requirement of validating passed information and allows duplicate EUIs within TTN (which might actually happen).
You might need to clear your webcache to load the latest javacript files.