Object reference not set to an instance of an object

[marius roma]

The report of a backup job says that the backup of a given VM failed with the following details:

Changed block tracking cannot be enabled: one or more snapshots present.
Creating snapshot
Error: Riferimento a un oggetto non impostato su un'istanza di oggetto.

The last line, in Italian, sounds like: "Object reference not set to an instance of an object".
The same job was executed till yesterday without errors.
I looked around the KB and the forum and found many pages reporting the same error, but the context is usually different.
How can I troubleshoot the error I get and solve it?
Regards
marius

[foggy]

How can I troubleshoot the error I get and solve it?

By providing full logs to technical support for investigation.