I tested the Plugin on the provided Sattelmühle and Menteroda .laz files and it´s working perfectly well :)
However, there seems to be an issue with the Solling_Test.laz file. When loaded into QGIS it gives the following error:
Ungültige Datenquelle: ~Solling_Test.laz ist keine gültige oder erkannte Datenquelle, Fehler "readers.las: Extra byte specification exceeds point length beyond base format length."
Testing the Plugin with the file gives: Konnte Quelllayer für POINTCLOUD nicht laden: ~Testdata/Solling_Test.laz nicht gefunden
Opening the file with Cloudcompare worked, so I assume the issue is with PDAL/QGIS interpreting the header.