- ad blocker
- content blocker
- third party cookies disabled
- cross site tracking disabled
These are known to cause problems for Readlang's authentication.
iOS Safari users should open the Settings app, look for Safari -> Privacy and Security, and ensure that the "Prevent Cross-Site Tracking" and "Block All Cookies" settings are both disabled.
(It's unfortunate that these steps are required, since there are privacy concerns, but they are required because Readlang works by injecting code to the web-site you are reading and that code needs to know which Readlang account you are signed in to.)