> hvað til saka hefir orðið með þeim
> what has been (the) causes with them

In other words, "what was the cause of their strife". More literally,
"what has happened by-way-of
offences/causes-[for-stife]/grounds-[for-a-quarrel] between them".

> "Hvað er nú að gert (past part indicating past rather than future, I

think)?" segir Flosi.

> 'What is (has been) now done regarding (it)?' says Flosi.

Yes, exactly.