Hello!
First, we need to undertand what is 500 error: The server could not acomplish the request.
So, I think that it is an InfinityFree security measure.
More info about 500 Error
https://en.m.wikipedia.org/wiki/List_of_HTTP_status_codes#500
More info about security measures
https://forum.infinityfree.com/t/ensuring-only-web-browsers-can-access-your-website/49353