There is code difference in determining the string value.

satellite 6.2.9 <<<----katello-3.0.0.125
satellite 6.2.10 <<<---katello-3.0.0.137


# diff candlepin.rb.629 candlepin.rb.6210
274,276c274,276
<             uri.port = proxy_config[:port]
<             uri.user = proxy_config[:user]
<             uri.password = proxy_config[:password]
---
>             uri.port = proxy_config[:port].to_s
>             uri.user = proxy_config[:user].to_s
>             uri.password = proxy_config[:password].to_s

Make the following change seems to fix the issue:

uri.port = proxy_config[:port].to_s if proxy_config[:port]
uri.user = proxy_config[:user].to_s if proxy_config[:user]
uri.password = proxy_config[:password].to_s if proxy_config[:password]

Manifest refresh worked both through UI and hammer:

hammer> subscription refresh-manifest --organization-id 1
[......................................................................] [100%]

So I am moving this to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2017:2466