I have the same issue.
We should be able to sync: xenial,xenial-updates,xenial-security,xenial-proposed,xenial-backports
all at once based the pulp-deb documentation.
When i sync Releases: xenial
I get a checksum did not match error as well: "PulpCodedTaskFailedException: Unable to sync 831513e4-8031-4dc0-b184-0b39ea4d4381 from http://archive.ubuntu.com/ubuntu: mismatching checksums for roundcube-core_1.2~beta+dfsg.1-0ubuntu1_all.deb: expected 2c07729047ebc9206038d90c88d77e392f68fcfc5a3b7cb30cfd6e7cf15157c3, actual 4a80f69bd8c68c737aebcfe53e458fc65205e8d06c1339aaa364c53db34787bf\n",
Here is the thread i created based on the issue i’m still having.