THIS IS 100% a PROXY ERROR
Make sure your configured correctly --> OpenProject - NGinX and Apache - Reverse Proxy
There is a OP issue, if you startup using an SSL Proxy in the frontend, OP defaults to http. You need to change the URL and Protcol under https://domain.com/settings.
To "trick" your browser, login once, when it doesn't proceed manually edit url to "/settings". Save, open new tab and test!
Also, make sure to check out my docker setup page for mounting the Apache directly and enabling SSL straight away,