Invalid geometry type after clipping

I have two files (.xyz) with point data from a terrain. I’d like to create contour lines of said terrain but in a smaller area (limited by a polygon). After importing the points in two layers, I merge them into one layer. Without any problems I’m now able to create contour lines using the contour plugin (version 2.0.8.).

But as mentioned before, I only want the contour lines in a specific area (also because of the file size). So I’m using the clipping tool provided by QGIS (vector-geoprocessing-clip) with the point layer as input and the polygon layer as overlay layer. That gives me a layer only with the points from that specific area. When I try to process the clipped layer I get the following error message:

Warning: Invalid geometry type for contouring – must be point
geometry.

Is anyone experiencing the same problem? I know that this problem didn’t pop up with older versions, as i was able process the exact same data with the exact same procedure without any problems. I’m using the following version of QGIS:

I’m fully aware that there are multiple workarounds available such ase "select by location". But working with big data I find it easier to have the selected area directly saved into a file.