Error processing personal DLs by CPUU. Error: "This group contains more members than are supported when using Microsoft Office Outlook 2007 Service Pack 1 or an earlier version.' of type 'One-off object'" (4259005)

When processing personal DLs by CPUU the following error is generated:

2/14/2020 8:05:06 AM 7348 [Trace] Preparing personal distribution list 'H - 2019 List' message for update.
2/14/2020 8:05:06 AM 7348 [Trace] Updating contact 'This group contains more members than are supported when using Microsoft Office Outlook 2007 Service Pack 1 or an earlier version.' of type 'One-off object'.
2/14/2020 8:05:06 AM 7348 [Trace] Number of members in personal distribution list is '1'.

The DL is quite small and also Outlook used is not 2007 version.