Hi @Dev314r, welcome to Nextcloud community 
Unfortunately a lot of information to help you is missing as you ignored the required support template. Please edit into your post and add as many information as possible. This will give us the technical info and logs needed to help you!
The problem sounds like often discussed misconfiguration related to reverseproxy and bruteforce protection. Make sure your reverse proxy settings are valid and the reverse proxy sends right headers. review 101: reverse proxy