GRASS r.slope.aspect via QGIS 2.18 vs QGIS 3.4 - not working

I seem to be unable to replicate actions with GRASS tools when I access them through QGIS 3.4. In particular the r.slope tool. I receive error messages and it fails in QGIS 3.4. They seem to work fine when accessed via QGIS 2.18. Is QGIS 3.4 known for such issues? I run Windows 10.

***2018-12-18T00:08:30     CRITICAL    Traceback (most recent call last):

          File "C:/PROGRA~1/QGIS 3.4/apps/qgis/./python/plugins\processing\algs\grass7\Grass7Algorithm.py", line 413, in processAlgorithm
          Grass7Utils.executeGrass(self.commands, feedback, self.outputCommands)
          File "C:/PROGRA~1/QGIS 3.4/apps/qgis/./python/plugins\processing\algs\grass7\Grass7Utils.py", line 411, in executeGrass
          startupinfo=si if isWindows() else None
          File "C:\PROGRA~1\QGIS 3.4\apps\Python37\lib\subprocess.py", line 756, in __init__
          restore_signals, start_new_session)
          File "C:\PROGRA~1\QGIS 3.4\apps\Python37\lib\subprocess.py", line 1155, in _execute_child
          startupinfo)
          OSError: [WinError 87] The parameter is incorrect***