Question

How do I search for Challenges, where I left a write note ?

Answer 1

In profile stats, there is a Challenge caches tab (for paying members), where you can see all the challenge caches with write notes that you did not log yet.

Comments

Thanks a lot. I've seen it before, couldn't find it again ! My Problem is solved !!!
Thank you for beeing so kind ! Your's sincerly    Manfred8bm

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This is entitled 'Challenges signed - but not yet qualified for' and is located below the challenges you have found
Which is not strictly correct - it pulls up all those which have write notes not just those you have signed
I know I have signed only 4 which I have yet to qualify - my list in the above tab shows 219

Answer 2

As an aside - and depending on what your write note is for :-
I ensure that any challenges that I have researched and qualify for I change the co-ords to 'amended' (so they show as italic) 
That way I can search for these (on GC website) with the filters
Found Status - 'unfound'
Geocache type - 'unknown',
Geocache name contains - 'challenge'  and
Corrected co-ordinates - 'Updated coordinates only' 

That gets your list of qualified for challenges 

You could additionally create a list for all of these