With some help from @neopch, I will update my earlier response with some images, @breakaway-dev. In the dialog, after ...
- Following the "Add Btc Profile" link
- Adding your existing Hive account name
- Successfully connecting your wallet, after clicking on "Get btc address"
- "Sign Message"
- "Connect to Hive Account
... this is what appeared on my screen:

This is what appeared on @neopch's screen (which, of course, is what one would expect):

But ... This then appeared on both of our screens
:

And ... Upon confirming, we both saw this confirmation
on our screens:

In his case, he was able to go on and successfully log in to the One BTC Club website. I was not ...
I hope that helps you. I had forgotten the error message, due to the fact it is quickly followed by the Confirm dialog in Hive Keychain. I assumed, obviously incorrectly, that this glitch did not matter, since ultimately I see confirmation that my Bitcoin address and signature successfully added to your Hive profile!
.
Nope. Not in my case. Whatever "status code 500" is, hopefully that will provide the missing link you need in helping resolve whatever the issue is preventing being able to log-in to the One BTC Club website.
RE: `OBC` development update