For a note taking app that focuses on security the https website doesn't work Google reports as un-safe and not secure.

Might have been a time-issue, the certificate is valid, and not just renewed:

Sometimes browsers give certificate errors if the time on your device and on the supplying server is (too) different.

Interesting… I wonder why it’s off so much? I don’t get that issue on other websites.

No idea, whois shows it’s being hosted on aws, so it’s not a weird corner of the 'net. But time-errors are horribly complicated to figure out as they come and go. If it keeps happening then there’s a config error somewhere, but otherwise it’s just solar-flares (old internet meme BOFH excuses · GitHub)