Hi Stefano,
It seems your filter syntax is wrong.
There is a very simple test, which I always do with my fingers. I add 1 for opening brace and remove on for a closing brace. In the end it needs to be 0.
This does not tell if the filter is valid, but it is a good indicator. If at the end there is a finger left, or “missing” 
I did test it. It should do what you want. See the start of the filter [all[current]... there is only 1 opening brace in front a “all”
<$list filter=[all[current]backlinks[]tag[Task]]>
<$link/>
</$list>
You may also have a look at talk.tiddlywiki.org, which is the new official forum now.
There is much more traffic so you may get your answer faster.
Have fun!
Mario