There’ll be no trouble, if by “trouble” you mean “streets on fire and riots” type trouble. I’ve just come back from ten weeks in Platsa, Messinia and the only trouble I had was the same as everyone else had, namely getting money from my bank account, due to bank closures and then withdrawal limits (currently €420 per week). I don’t think that this applies to withdrawals on a non-Greek account though.
Best bet is just take cash with you – incidentally, it wouldn’t make any difference being German, I know plenty of Germans living in Greece and no one treats them any differently.
I haven’t been to Kefalonia for 10 years, I keep meaning to pop over from Kilini to Poros for a few days to see a few old friends but never seem to actually do it. Maybe this autumn.