Is the front-end user logged in to the other context?
Are you using this in the Login snippet tag:
&contexts=`web,otherContext`
You are heroes!
Thanks for the almost simultaneos answers, here an in the modxcms.de forums from BobRay, Bruno and Markus1.
Setting &contexts explicitely is the solution even if the user is logged in in the same context.
best regards
manu