Will87

OK, if you say so. But here is my problem: This error appeared on both my test and production website, immediately after updating wordpress download manager 2.6.93. If I revert back to 2.6.93 – the error goes away again. I have also tried disabling all the other plugins, with no success. How can we resolve this problem ?

I know what you mean, I tried that too and the error remained. But as I said, when I rolled back the version to 2.6.96, the error disappeared.
I switched on debug briefly, here is an excerpt from the debug.log file – the PHP warnings seem to refer to plugin.php on line 214 (which is the warning visible on the website frontend), together with download manager. Can you please check again, or suggest a fix?

[19-Dec-2014 20:19:53 UTC] PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /var/sites/t/thetheory.works/public_html/oldsite/wp-includes/plugin.php on line 214
html/oldsite/wp-content/plugins/download-manager/download-manager.php on line 15

Hello

I still have the error (though the line reported is now 214)

I rolled back the Download Manager version to 2.6.96 and the error message disappeared. When I updated to the latest version (2.7.8), the problem reappeared. Let me know if you have time to look at the test website, and I will pass on details.