SUSE Manager Inter-Server Synchronization (ISS) ending with error: null value in column "checksum" violates not-null constraint

# satellite-sync -c pfs3_3-stable-sle-product-sles15-sp1-pool-x86_64
> 11:57:35 SUSE Manager - live synchronization
> 11:57:35    url: https://XXX.YYY.ZZZ
> 11:57:35    debug/output level: 1
> 11:57:35    db:  susemanager/<password>@susemanager
> 11:57:35
> 11:57:35 Retrieving / parsing orgs data
> 11:57:35 orgs data complete
> 11:57:35
> 11:57:35 Retrieving / parsing channel-families data
> 11:57:35 channel-families data complete
> 11:57:35
> 11:57:35 Retrieving / parsing product names data
> 11:57:35 product names data complete
> 11:57:35
> 11:57:35 Retrieving / parsing arches data
> 11:57:35 arches data complete
> 11:57:35
> 11:57:35 Retrieving / parsing additional arches data
> 11:57:35 additional arches data complete
> 11:57:35
> 11:57:35 Retrieving / parsing channel data
> 11:57:36    p = previously imported/synced channel
> 11:57:36    . = channel not yet imported/synced
> 11:57:36    base-channels:
> 11:57:36       . sle-product-sles15-sp1-pool-x86_64    3       full import from Thu Oct  8 13:21:18 2020
> 11:57:36
> ERROR: Encountered IntegrityError:
> null value in column "installer_updates" violates not-null constraint
> DETAIL:  Failing row contains (120, null, 1, 513, sle-product-sles15-sp1-pool-x86_64, /dev/null, SLE-Product-SLES15-SP1-Pool for x86_64, SUSE Linux Enterprise Server 15 SP1 x86_64, null, null, Y, null, null, null, null, 3, N, 2020-11-03 09:57:36.338362-01, null, null, private, null, null, null, null, SLE-Product-SLES, 2020-11-03 09:57:36.338362-01, 2020-11-03 09:57:36.338362-01, null).
> consider removing satellite-sync cache at /var/cache/rhn/satsync/* and re-run satellite-sync with same options.
> If this error persits after removing cache, please contact SUSE support.

The following command should be run on the Master, just for reporting purposes (in case further help from Support is needed at a later point of time):
 

grep -v rhn-channel-installer-updates /var/cache/rhn/xml-channels/*xml

After that, the cache should be deleted, and the apache service restarted (commands to be run on the master):
 

rm -rf /var/cache/rhn/xml-*
capache2 restart
 

The same applies to the slave:
 

rm -rf /var/cache/rhn/satsync/*
rcapache2 restart
 

They sync should be possible after this. If symptoms should still be present, run the following commands both on the master and on the slave and report the results to Support:
 

spacewalk-sql --select-mode - <<<"SELECT id, label, installer_updates FROM rhnChannel;"
rpm -qi spacewalk-backend

Chances are a master-side cache still exists with old entries.