The reason is not the custom-ga-command as it would just return the reply
without trying to parse it. Libvirt calls guest-get-disks itself via a
supported API and fails to parse the response from Windows.

virJSONValueObjectGetStringArray originally didn't report errors but an error was added when the function was reused further, not considering the original use.

Reproduced this bug with:
libvirt-8.0.0-5.5.module+el8.6.0+16828+96e76c36.x86_64

1. prepare a win guest with guest agent device
# virsh domtime win2k19 
Time: 1669923955

2. config the libvirtd conf file
# grep -v ^# /etc/libvirt/libvirtd.conf 
log_outputs = "3:file:/var/log/libvirt/libvirtd.log"

3. restart libvirtd

4. get guest disk info
# virsh qemu-agent-command win2k19 '{ "execute": "guest-get-disks" }'
{"return":[{"name":"\\\\.\\PhysicalDrive1","partition":false,"address":{"serial":"QM00003","bus-type":"sata","bus":0,"unit":0,"pci-controller":{"bus":-1,"slot":-1,"domain":-1,"function":-1},"dev":"\\\\.\\PhysicalDrive1","target":0}},{"name":"\\\\.\\PhysicalDrive0","partition":false,"address":{"serial":"QM00001","bus-type":"sata","bus":0,"unit":0,"pci-controller":{"bus":-1,"slot":-1,"domain":-1,"function":-1},"dev":"\\\\.\\PhysicalDrive0","target":0}}]}

(There is no dependency element in win guest output)

5. check the log
# cat /var/log/libvirt/libvirtd.log 

2022-12-02 08:48:08.888+0000: 34725: error : virJSONValueObjectGetStringArray:1324 : internal error: dependencies is missing or not an array
2022-12-02 08:48:08.888+0000: 34725: error : virJSONValueObjectGetStringArray:1324 : internal error: dependencies is missing or not an array

6. prepare a rhel guest, and get the disk info
# virsh qemu-agent-command rhel8 '{ "execute": "guest-get-disks" }'
{"return":[{"name":"/dev/vda1","dependencies":["/dev/vda"],"partition":true},{"name":"/dev/vda2","dependencies":["/dev/vda"],"partition":true},{"name":"/dev/vda","dependencies":[],"partition":false,"address":{"bus-type":"virtio","bus":0,"unit":0,"pci-controller":{"bus":4,"slot":0,"domain":0,"function":0},"dev":"/dev/vda","target":0}},{"name":"/dev/dm-0","dependencies":["/dev/vda2"],"partition":false,"alias":"rhel-root"},{"name":"/dev/dm-1","dependencies":["/dev/vda2"],"partition":false,"alias":"rhel-swap"}]}

(There are dependency elements in rhel guest output)

In the function of
int qemuAgentGetDisks(qemuAgent *agent,
                      qemuAgentDiskInfo ***disks,
                      bool report_unsupported)
...
        if (virJSONValueObjectGetBoolean(entry, "partition", &disk->partition) < 0) {
            virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                           _("'partition' missing in reply of guest-get-disks"));
            goto error;
        }

        disk->dependencies = virJSONValueObjectGetStringArray(entry, "dependencies");
        disk->alias = g_strdup(virJSONValueObjectGetString(entry, "alias"));
...

libvirt tries to parse the dependencies info no matter whether dependencies info contained in qemu agent command outputs.

Fixed upstream by:

commit 3b576601dfb924bb518870a01de5d1a421cbb467
Author: Peter Krempa <pkrempa>
Date:   Thu Dec 1 17:02:42 2022 +0100

    qemuAgentGetDisks: Don't use virJSONValueObjectGetStringArray for optional data
    
    The 'dependencies' field in the return data may be missing in some
    cases. Historically 'virJSONValueObjectGetStringArray' didn't report
    error in such case, but later refactor (commit 043b50b948ef3c2 ) added
    an error in order to use it in other places too.
    
    Unfortunately this results in the error log being spammed with an
    irrelevant error in case when qemuAgentGetDisks is invoked on a VM
    running windows.
    
    Replace the use of virJSONValueObjectGetStringArray by fetching the
    array first and calling virJSONValueArrayToStringList only when we have
    an array.
    
    Fixes: 043b50b948ef3c2a4adf5fa32a93ec2589851ac6
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2149752
    Signed-off-by: Peter Krempa <pkrempa>
    Reviewed-by: Michal Privoznik <mprivozn>

commit 6765bdeaf7e9cbdb4c39d47f3b77fb28a498408a
Author: Peter Krempa <pkrempa>
Date:   Thu Dec 1 13:32:07 2022 +0100

    util: json: Split out array->strinlist conversion from virJSONValueObjectGetStringArray
    
    Introduce virJSONValueArrayToStringList which does only the conversion
    from an array to a stringlist.
    
    This will allow refactoring the callers to be more careful in case when
    they want to handle the existance of the member in the parent object
    differently.
    
    Signed-off-by: Peter Krempa <pkrempa>
    Reviewed-by: Michal Privoznik <mprivozn>

v8.10.0-32-g3b576601df

Verify this bug with:
libvirt-8.0.0-12.module+el8.8.0+17545+95582d4e.x86_64

The first 4 steps are the same with those in Comment #3

5. check the log
# cat /var/log/libvirt/libvirtd.log 
(no output)

As the testing result matches with the expected result. Mark the bug as verified.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (Moderate: virt:rhel and virt-devel:rhel security, bug fix, and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2023:2757