PHP Warning: Couldn't set response type to 404 because of output on line 326 of

Questions
1

Silverstripe Version:
4.3.3
Question:
I get a server error after I enter my login and password at /admin- how do I fix this?
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seeing the following in the application error log
PHP Warning: Couldn’t set response type to 404 because of output on line 326 of /home/carbery1/public_html/vendor/silverstripe/framework/src/Control/Session.php in /home/carbery1/public_html/vendor/silverstripe/framework/src/Control/HTTPResponse.php on line 397

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2

A warning shouldn’t stop execution. Have you got your site in dev mode? If not try changing your .env file to dev and see if you get a more helpful error?