It's hard to tell, it looks like there was no URL set with the request. Can you hover the Visitor-Id and click on the export icon that should appear next to it? There you will see more details. Is a page title set? Can you maybe try to reproduce the steps?

hmm in visitor log I haven't seen this before, but the Visits widget looks the same in our installation.
Maybe it's fine this way and I didn't get what it wanted to show?

@tsteur below what I see in exported XML

<row>
<type>action</type>
<url/>
<pageTitle/>
<pageIdAction/>
<serverTimePretty>Sun 4 Oct 04:33:29</serverTimePretty>
<pageId>1634354</pageId>
<timeSpent>1</timeSpent>
<timeSpentPretty>1s</timeSpentPretty>
<icon/>
<timestamp>1443933209</timestamp>
</row>

And I cannot reproduce this :(

How often does it occur? Was it only once? I'm not 100% sure whether pageId refers to log_action table but in case you have access to the database, can you run the following query?

SELECT * FROM piwik_log_action WHERE idaction = 1634354;

If you use a different database table prefix you might have to remove piwik_ after the from or replace it with something else.

FYI: I just had a similar case and it happened when there was a download without an absolute URL. Eg I did something like _paq.push(['trackLink', '/download.pdf', 'download']). This resulted in Page URL not defined. After adding a domain it worked

Added to our FAQ What is “page url not defined” in the Pages report?

the following:

3. if you manually track file downloads using _paq.push(['trackLink', 'https://mydomain.com/download.pdf', 'download']) for example, note that you must use a full valid URL.

if you have more info what was the problem @Globulopolis let us know, we can add to FAQ