As already mentioned, a difficulty of 0.000 means you're probably on a fork.
Your already payed out reward will disappear if you get to the correct chain, but don't worry, all will be payout out on your next stack since you haven't stacked on this chain.
Probably 90% of the network (I really don't know the real percentage, but the net weight is way to low, so no coins are online on this chain) is currently on multiple forks, so it's really easy to stack if you're on the right chain.
As far I know I'm on the longest chain:
Block 1149773 (hash: eb5e26a49311cc8b3b49438aae193fddfff8a3def994061adf460b45c1e4572c)
Difficulty 0.035
Net Weight 195690.
It took 5min after opening the wallet to stack with 4k GRC (last stack was 5 days ago, which was on a low difficulty chain too).
Thanks for a lot of useful info! I don't know what's going on with this fork thing.
After receiving this stake, it shows me 1-2 minutes or even seconds to receive next reward, but it's not coming.