AssertionError: Could not retrieve user info for user xxx, server returned: {}
This error seems to occur if there is a mismatch between your username and the username saved by PsychoPy. The easiest option is to close PsychoPy, delete the configuration file to clear the saved login data, and try again.
On a PC, to get to your psychopy configuration files, type this into the Windows Explorer path:
%appdata%
In my case that takes me to C:\Users\morys\AppData\Roaming\ but the location depends on your username and how your computer has been configured (roaming profiles, versions of Windows etc). From this folder go to psychopy3/pavlovia and delete the file users.json.
On a Mac, you can try the following in command line to find the configuration files.
cd ~/.psychopy3
open .
Alternatively, open Finder and follow the steps Go > press option or alt key (to show Library) > Library > Preferences and delete the file org.opensciencetools.psychopy.