Unable to reach www.t-online.de over IPv6 since late September

For 42 days we have not been able to reach www.t-online.de over IPv6.

C:\>tracert -6 www.t-online.de

Tracing route to www.t-online.de [2003:2:2:40:62:153:159:92]
over a maximum of 30 hops:

  1 4 ms 3 ms 2 ms router-core.mtcnet.net
  2 3 ms 3 ms 2 ms sxct.sxcy.mtcnet.net
  8 24 ms 23 ms 22 ms sl-crs2-chi-te1-2-0-1.v6.sprintlink.net
  9 31 ms 30 ms 30 ms sl-crs2-akr-bu-2.v6.sprintlink.net
10 41 ms 41 ms 39 ms sl-crs2-rly-bu-2.v6.sprintlink.net
11 40 ms 40 ms 39 ms sl-crs2-dc-po0-9-0-0.v6.sprintlink.net
12 42 ms 42 ms 41 ms sl-st31-ash-po0-2-0-0.v6.sprintlink.net
13 * * * Request timed out.
14 * * * Request timed out.
15 * * * Request timed out.

DT (Deutsche Telekom) is not learning our routes (AS53347) or the routes of
our upstream provider (AS5056). DT's looking glass
(https://f-lga1.f.de.net.dtag.de/index.php?pageid=lg) confirms that:

  sh ipv6 bgp 2607:fe28::/32
  % Network not in table

  sh ipv6 bgp 2001:05f8::/32
  % Network not in table

Our prefix goes over our upstream provider's transit link with Sprint, so on
September 26 our upstream provider opened up a ticket with Sprint. From
what our upstream provider tells us, Sprint is advertising our routes to DT
but DT has an IPv6 prefix that is blocking all of Sprint's customer
prefixes. It's almost been a month since the last substantive update from
DT (via our upstream provider, via Sprint).

If anybody is able to help and shake something loose at DT, that would be
appreciated. For the right people, I can provide both Sprint and DT's
ticket numbers.