Same here, didn’t find a solution for now.
I have the same crazy number of logs
Thu Jan 7 10:28:03 2021 daemon.warn dnsmasq[3494]: possible DNS-rebind attack detected: **URL**
NextDNS stop working after ~6h, sometimes even faster (I experienced it after only 1h).
Set DNS Rebinding Protection to OFF didn’t work for me.
Note: I already had the same problem with the previous firmware update (3.104 w/ Stubby setup).
In a previous post from @MRizkBV said it was from the use of Stubby?