I still struggling to ID this bird as a Reed Warbler with confidence.
First impression was indeed a Melodious Warbler, because jizz was good, as was overall colours (such a pale sandy-grey back is still within variation for this species, espacially when the taken into account that the picture seems slightly overexposed) including an appearent yellowish throat and eyering (and the right shade for Melodius). Because the others havent seen such a yellow colour, I looked at this bird on different (=better) screens, and it isnt there on some.
Such pale flesh coloured legs are very unusual for Reed. I have seen other, obvious sick, ill passerines with a similar leg colour before, so I think this must be also possible for Reed. But its easy within variation for Marsh. For an example see the attached pictures (Marsh warbler, Schwetzinger Wiesen, SW-Germany, 5.7.2019).
When I first saw this picture, I thougt the real impression of the loral area was somehow distracted by displaced/ruffled feathers and/or "JPEG/photographic" artefacts, but after a second look I agree with all others, that the loral area is unusual for a hippolais-warbler.
I asume that Marsh is a rarity in Valencia, but I think if more pictures are available, this bird might well turn into an IDable Marsh Warbler, even in a vagrant context.