The following signatures were invalid: EXPKEYSIG 1397BC53640DB551

This is the protection you are getting from these checks. You don't want to update your software right now while something is messed up on Google's end. Wait until they fix it. Don't try to override by reinstalling keys until some official word comes out that a new key is the solution.

Apparently Google did not extend the validity of the signing cert... it was due to end today and so it did. https://pgp.surfnet.nl/pks/lookup?op=vindex&fingerprint=on&search=0x7721F63BD38B4796

maybe Google will change it, today or so… then the update of the cert should work fine and everything should go back to normal.

the problem was solved by Google Abr 12/2019 (Only Google Chrome. Tested in Ubuntu 18.04.x)

There's nothing to do. The repository has already been signed

Update apr 19/2019:

Google Team has confirmed that additional fixes have gone out for other non-Chrome Google products

source: https://support.google.com/chrome/thread/4032170