Wanted to clarify my last statement a little bit. 

I know Elgg 3 is working with fresh MariaDB. My issue is when I migrated existing MySQL data to MariaDB.

http://learn.elgg.org/en/stable/design/events.html#register-to-handle-an-elgg-event is only Elgg core events' list.

You can use elgg_trigger_event() function in your action and register any callback using elgg_register_event_handler() function.

Thanks

I don't understand it.

Should I disable/enable 'Friends' or 'Invite friends' plugins?

And what result do you want to see?

With both 'Friends' and 'Invite Friends' disabled: http://336.waw.pl/3-3-16_bug_a.png

This doesn't appear in Elgg 3.3.16 with the bundled plugins.

Disable your 3rd party activated plugins and check your site again.

Solution

Following up -

The problem is fixed. I disabled all non-bundled plugins, enabled the plugin on its own, it worked. I re-enabled the plugins and it still worked. I can't make it fail any more.

I am mystified.

The only thing I can think that it must have been is a cache issue, but caches are disabled via developer tools plugins. Unless there's a cache on the webserver, perhaps, but the server has been restarted since the problem occurred, so that seems unlikely.

Jon

RSS Import uses Simplepie library which creates cached RSS requests and stores them in the /data directory.

So...

The only thing I can think that it must have been is a cache issue, but caches are disabled via developer tools plugins

This could be the reason of your issue

It's not about TidyPics or Blog plugins but File plugin.

Every time you upload a file, an event appears in the activity.

It doesn't matter where you do it when creating a blog or when uploading a file normally.

I haven't found a way to do this using Elgg API (none of the hooks I know work for files).

Therefore, I can advise to use the custom view:

/mod/my_plugin/view/default/river/object/file/create.php

With empty code:

<?php

You can take my plugin from here so as not to create a new one and put the above file there.

Thank you, Nikolai. Much appreciated.