XF is emitting a <base> tag that points to the root of the forum on every page, breaking same-document URLs. Full friendly URLs are enabled. Here's an example value of $requestPaths from a visit to /members/ (domain anonymized):
It looks like the basePath and fullBasePath are wrong. Let me know if you need more info for debugging.
host: example.com protocol: https basePath: / fullBasePath: https://example.com/ requestUri: /members/ fullUri: https://example.com/members/