This was tested with Ekos Windows and Linux. Ekos versions tested were Bleeding and Nightly #171 and later. When the astrometry server is changed in the settings (e.g. in my case from astrometry to ansvr and back), Ekos doesn't try to authenticate after the server change. So if you use ansvr, changing to astrometry fails because Ekos uses the ansvr session id in the astrometry.net API requests. Changing from astrometry to ansvr works because ansvr doesn't actually care about user and session id.
This is now handled by StellarSolver starting from 3.5.0