I have a relatively new Nexcloud installation(a few weeks old), which I installed without any Apps/Addons, etc
I then decided to try Snappymail, which seemed to go without a hitch(fr./apps panel) - though upon trying to login w/ email:password, I get the following;
You seem to have ignored the support template and didn’t provide any valuable information about your environment, configuration etc. so it won’t be possible to give you any advice except to check your Nextcloud configuration.