Nils, I would think Green Sandpiper is quite a rarity in your part of the world? The opposite is true here in the UK with Solitary being a major rarity.
If I saw your bird here it would stand out as unusual for a Green Sandpiper, looking quite elegant & long-legged, but I'd caution against drawing firm conclusions from a single image. We know that the same bird can look both 'compact' then 'slim and elegant' in different images! so we have to be mindful of that if you only have this one. Also, your bird seems to lack the more contrasty dark & white impression that Green Sandpiper usually gives.
I found quite a number of photos of Solitary showing 'too few' tail bars (according to how field guides depict them). In the attachment there are at least 3 birds that are comparable to yours in terms of this feature.
On your bird, the most striking feature on the face is the bold white eye-ring, which is at least equally noticeable to the white supercilium (over the loral area). This is typical for Solitary, whereas on Green, usually the bolder white loral line (joined to a thinner eye-ring) stands out the most.
From one image, it's hard to make strong conclusions one way or another, but your bird looks more typical for Solitary to me, in terms of its overall look or 'jizz'.
Sneaking out from beneath the folded wing there appears to be some flank-barring, which is a Solitary feature, but it isn't always visible in photos. This should favour Solitary over Green.
The only really 'wrong' feature I can see is that the primary projection should be longer in Solitary, extending beyond the tail, but what if this bird is in moult and has lost these feathers?
Other subjective features like different calls and dark underwing would really need to be backed up with photos and recordings of call as both species could be described as having dark underwings, and their calls are quite similar too.
I'm not saying your bird isn't a Green Sandpiper, but I don't see enough features to point away from Solitary just yet (but there are some unanswered questions...?) Is this really the only image you have?
