Login Problem

For some reason, everytime someone logs into my site, they get the following prompt when the next page loads:

Not Found
Apologies, but the page you requested could not be found. Perhaps searching will help.
(And there’s a search box underneath this message)

Does anyone know why this is happening and how I can fix it? I do have this redirect/login plugin installed, so many that’s the culprit? I would really be interested to hear your feedback. Thanks!!