Hello! I just installed Nextcloud (using Softaculous) on my test account and I can replicate the issue of not being able to login. I've flagged it in our staff channel, along with the error logs I'm seeing.
Here is a link explaining how to view error logs on your account, I figured I'd share it since it's just good to know how to do anyway: https://wiki.helionet.org/View_Error_Logs
If you do have a look at your logs, if you see anything different than the errors I get below, please let us know. But for right now I think it's safe to say it's not an issue isolated to your account, so we'll look into it and post more updates when we can.