I have seen to be having the same issues on both of my running dietpi configurations:
RP4 and RP3B+
I install diet pi, i get to the whole process of changing the passwords for both accounts root, and dietpi. I then proceed to update the system and install additional software (pi-hole). When I then try to SSH into using any software it claims the password to be wrong for either account, I then try to login into my xxx.xxx.xxx.xxx/admin for my pihole and the same deal, it says “wrong password.” The funny thing here is if I reset the pihole password pihole -p -a and leave blank I can easily access the admin page, but otherwise regardless of what password I change it to, it gets decline. I’m not sure what am I doing wrong on both installations, but I cannot seem to find and answer anywhere. I can only login to dietp if I have a screen on and thats it. The only thing I’m disabling is the console when prompted.
Any help is appreciated.