Return package (with the file extension "wsxz" only) cannot be opened both in Trados Studio 2019 and 2021

Return packages with extension "wsxz" cannot be opened both in 2019 and 2021, although the project packages with extension "wsxz" can be opened both in 2019 and 2021. 

Only the "...sdlrpx.wsxz" files (received via WorldServer) get stuck. ".sdlrpx" files (received via TMS) can be normally opened.