As a workaround you can enable "Use HTTP streaming for channels" in HTSP client addon settings.
Enabling http streaming didn't really help, I still have a black screen for SD channels with http streaming and H/W acceleration enabled. Without acceleration I get both sound and video for HD and SD streams.
However I found one SD channel that did not have video or sound without http streaming which started working with http streaming... (sat1)
Did you find any clue in the logs I provided chewitt?
Should I provide logs for sat1 with and/or without http streaming (and hw acceleration disabled)?
I'm still not sure if the issue is causes by Kodi, tvheadend or may even be caused by something else. If anyone here can blame something specific I also wouldn't mind filing a bug report against that.