It is currently not possible to sign in to Boxcryptor for Chrome due to Google's distrust of Symantec certificates. For communicating with our API we're using a certificate from Thawte (subsidiary of Symantec) which the new Chrome version 66 released in April does not trust anymore and thus a connection to our servers cannot be established. Only the Chrome plugin is affected, because only Chrome does not trust this certificate anymore - all our other clients are not affected by Google's decision.
We're currently in the process of replacing the certificate and will switch over to the new certificate in the next few days. After this switch, the Chrome plugin will continue to work again.
You can read more about Google distrusting Symantec here: https://security.googleblog.com/2018/03/distrust-of-symantec-pki-immediate.html