Having trouble creating categories: call_user_func_array() in plugin.php

Hi,
I need some experts advice:

I’m using wordpress with the “portfolium” theme from https://wpshower.com/ but the problem also occurres when I use a default wordpress theme.

Here my problem: Whenever I try to create a category in the theme, the following error occurres:

Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘wp_ajax_add_tag’ was given in /homepages/xx/xxxxxx/htdocs/wp-includes/plugin.php on line 406
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I deactivated all plugins and also searched for the add_filter line in the functions.php, that some people advise to comment out…I don’t’ have a add_filter in there…

Here is what line 406 in plugin.php says:

do {
foreach ( (array) current($wp_filter[$tag]) as $the_ )
if ( !is_null($the_[‘function’]) )
call_user_func_array($the_[‘function’], array_slice($args, 0, (int) $the_[‘accepted_args’]));

} while ( next($wp_filter[$tag]) !== false );

Does anybody know the mistake? Which callback is the invalid one?
Thank you very much for helping, everyone!

flo