Hi Jérémie,
Indeed there are many constraints which have to be fulfilled when generating a mesh, and especially when dealing with quadrangular meshes.
When using the default 'Algebraic box method', you are not allowed to have more than 4 nodes along the arcs, each of them defining the 4 sides of the quadangular area. So if you have intermediate nodes (instead of vertices), that could be the problem.
Another problem can raise when drawing meanders with high curvature: in that case it may result in some quadrangular elements attempting to cross the meander, outside the quadrangular area, which is not accepted. So you can try to reduce the curvature and see it's better. Alternatively you may try to split a long quandarangular area into smaller areas (by adding arcs to cut the polygon into multiple parts). That may help to avoid odd mesh geometry in meanders.
The "Transfinite interpolation" method wasn't available in older versions of the Mesh Generator, so I suppose you are using one of these versions!
Best regards
Mathieu