Anymore hints? Otherwise I'll try to reinstall everything... And maybe switch to LE9
In that case I would test whether the situation is improved in the LE9 millhouse builds.
I have two MyGica T230 sticks too which work just fine for me under LE 8.2.3 on my Generic machine, though I use DVB-T2 rather than DVB-C which could be the difference.
Presumably, you've only enabled DVB-C in the TVHeadend adapter settings and have not enabled DVB-T?