Hello, I had similar problem, and found finally solution in case you are using service of OVH. If you are using webhotel to host Flarum, above WWW folder there is .ovhconfig file. First line tells engine.
Usually, it's for example like this:
Change it to this:
CGI is usually common interface for web applications. Probably this wasn't default(Possibly CLI?), and caused then some errors.
At least it worked for me. No Oops again in log in.