When running AdminUI, when viewing a Partnership, in the Signature field, the expiry date shows nothing, no date.
The "Region Settings" of the server hosting the AdminUI (Windows Server 2016) had changed from the default (Location "United State") to Italy.
This generated the error when modifying the Partnership certificates.
Once the "Region Settings" was restored to default, it was possible to resume operations but only with browsers set in English.
Investigate with the Operating System vendor to sort out this issue.
There's no recent change in the language used to browse the AdminUI. configurations are related to the OS settings. Applied the very last patches on the Laptop Windows OS as well as for the AdminUI Windows OS.
Note that the AdminUI has been fully tested with some browsers only (1).