At first I was thinking this is maybe related to #11900 but might not be as it seems to work in the last example. It seems to be related to eg typo in the segment, or some configuration setting regarding but seems unlikely as it works in the last case.

Does http://localhost:8080/index.php?module=API&method=Events.getCategory&idSite=6&period=range&date=2018-01-01,2018-01-31&segment=eventCategory==industrikatalog-bog-nr-4-jop4tn&format=json&token_auth=9ee59c92cc717332a56d0fed157ac54c return results?

@tsteur thank you for your reply.

No, the URL do not return any results.
I'm also storing other eventAction:

But any of them works.

I have plenty of event category, but it looks like if only the special-styrkerne-i43qh9 is able to return a data.

As you see - I do not use any segment.

Everything was working fine before. I think it stopped when I was playing with creating a custom segment. And this happening not only for my local environment, but also on my production server ;/

One more thing

Not working:
http://localhost:8080/index.php?module=API&method=Events.getCategory&idSite=6&period=range&date=2018-01-01,2018-01-31&segment=eventAction==openPDF;eventCategory==industrikatalog-bog-nr-4-jop4tn&format=json&token_auth=9ee59c92cc717332a56d0fed157ac54c

Working:
http://localhost:8080/index.php?module=API&method=Events.getAction&idSite=6&period=range&date=2018-01-01,2018-01-31&segment=eventAction==openPDF&secondaryDimension=eventCategory&filter_pattern_recursive=industrikatalog-bog-nr-4-jop4tn&expanded=1&format=json&token_auth=9ee59c92cc717332a56d0fed157ac54c

Main difference is as you see the second API use &method=Events.getAction instead &method=Events.getCategory. Of course the two methods return a different data structure, but subtable property is what I'm looking for.
In above example I used eventAction==openPDF, but this works as well for other eventAction's.

This issue may be fixed now in 3.12.0-rc: https://matomo.org/faq/how-to-update/faq_159/
See #14761 #11900 - could you maybe check and let us know @bluemanos ?

Closing as it should meanwhile be fixed.