Answer
Basic Authentication isn't supported on IFS Cloud's 'main' API endpoint — connections through main need OAuth. If a tool genuinely can't be configured for OAuth and Basic Authentication is unavoidable, the projection has to be accessed through the 'int' endpoint instead, and specifically needs to be added to the (legacy) Compatibility Application Type list first. Enabling Compatibility Application Type for a projection is a real network-security tradeoff though (it reintroduces Basic Auth for that projection), so it should only be turned on selectively, after weighing that tradeoff, rather than as a default fallback whenever OAuth setup feels inconvenient.