Can't add URL to webhook URI.

Thanks for the report! It looks like we may be having trouble connecting to your webhook URI with your TLS configuration. We'll look into it. 

As a few potential workarounds, can you try:

• resolving the TLS issues for your server noted on: https://www.ssllabs.com/ssltest/analyze.html?d=www.tarrison.com&hideResults=on
• removing the use of the port number
• failing those, just use http:// for now 

The reported error would be: "unable to get local issuer certificate". Trying directly in curl looks like:

$ curl -v "https://www.tarrison.com:8443/webhook/dropbox?challenge=6TV4oooNb9AnHM5vEK6qQw_yXE1dNNWQrcau-UY_YO8"
*   Trying 35.182.122.11...
* Connected to www.tarrison.com (35.182.122.11) port 8443 (#0)
* found 148 certificates in /etc/ssl/certs/ca-certificates.crt
* found 594 certificates in /etc/ssl/certs
* ALPN, offering http/1.1
* SSL connection using TLS1.2 / ECDHE_RSA_AES_128_GCM_SHA256
* server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
* Closing connection 0
curl: (60) server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
More details here: http://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.

That seems to indicate an issue with the certificate or chain your server is being served with. The SSL Labs test does indicate some certificate/chain issues, so I recommend resolving those, or switching to a different certificate entirely.