Repository Browser does not like tags with Debian epochs (gbp defaults)
Most likely an upstream Gitlab bug, but probably only frequently used in Debian repos.
git-buildpackage by default creates tags for Debian versions containing epochs by escaping the ':' with '%', for example in
Selecting the debian/1%13.18.3_dfsg-1 tag in the dropdown refers to https://salsa.debian.org/pkg-voip-team/asterisk/tree/debian/1%2513.18.3_dfsg-1, which displays the Gitlab standard 404 error page. There are also tags that respond with "400 Bad Request", i.e.