zypper commands return Error code: HTTP response: 0

When running zypper commands, an Error code: HTTP response: 0 with empty Error message is returned like in the following example:

susemanager:~ # zypper ref -s
Refreshing service 'Basesystem_Module_15_SP2_x86_64'.
Problem retrieving the repository index file for service 'Basesystem_Module_15_SP2_x86_64':
Download (curl) error for 'https://scc.suse.com/access/services/1931/repo/repoindex.xml?cookies=0&credentials=Basesystem_Module_15_SP2_x86_64':
Error code: HTTP response: 0
Error message:

Check if the URI is valid and accessible.
Skipping service 'Basesystem_Module_15_SP2_x86_64' because of the above error.
Refreshing service 'Python_2_Module_15_SP2_x86_64'.
Problem retrieving the repository index file for service 'Python_2_Module_15_SP2_x86_64':
Download (curl) error for 'https://scc.suse.com/access/services/1977/repo/repoindex.xml?cookies=0&credentials=Python_2_Module_15_SP2_x86_64':
Error code: HTTP response: 0
Error message:

Looking at /var/log/zypper.log, different HTTP error codes are reported as well:

grep "Error code: HTTP" /var/log/zypper.log | awk '{print $7,$8,$9,$10,$11}'|sort|uniq
Error code: HTTP response: 0
Error code: HTTP response: 407

On SUSE Manager, the repository sync may be interrupted at the same and the respective log files in /var/log/rhn/reposync report errors similar to:

2021/05/25 04:25:04 +02:00 Command: ['/usr/bin/spacewalk-repo-sync', '--channel', 'sle-module-basesystem15-sp2-updates-x86_64', '--type', 'yum', '--non-interactive']
2021/05/25 04:25:04 +02:00 Sync of channel started.
2021/05/25 04:25:06 +02:00 RepoMDError: Cannot access repository. Maybe repository GPG keys are not imported
2021/05/26 04:26:29 +02:00 Command: ['/usr/bin/spacewalk-repo-sync', '--channel', 'sle-module-basesystem15-sp2-updates-x86_64', '--type', 'yum', '--non-interactive']
2021/05/26 04:26:29 +02:00 Sync of channel started.
2021/05/26 04:26:30 +02:00 RepoMDError: Cannot access repository. Maybe repository GPG keys are not imported.

When looking at the reposync log files it is noticed the sync process used to work before but reported this error only lately.

Executing and extracting a supportconfig on the affected system might help to identify issues like this. In the extracted archive, see the file updates-curl-trace_*.txt for a curl trace of an HTTP connection. Look for any HTTP 40x replies that indicate an error condition. Here are some examples that have been found in such a condition:

0000: HTTP/1.1 403 Forbidden
[...]
== Info: The requested URL returned error: 403

or


0000: HTTP/1.1 407 Proxy Authentication Required
[...]
== Info: Authentication problem. Ignoring this.