Cannot change session name when headers already sent

Hello every one,

I am doing a bit change in the app registration, But it is unfortunatly that the server has an error after programming.
I guess that it an error about session I have to restart the server. But I am not sure about this solution.
The error is:

Der Server konnte die Anfrage nicht fertig stellen.

Sollte dies erneut auftreten, sende bitte die nachfolgenden technischen Einzelheiten an Deinen Server-Administrator.

Weitere Details können im Server-Protokoll gefunden werden.

Technische Details
Entfernte Adresse: 10.3.x.xx
Anfragekennung: YWWT61AQNVd52ebaeuY5NgAARBk
Typ: ErrorException
Code: 0
Nachricht: session_name(): Cannot change session name when headers already sent
Datei: /var/www/nextcloud/lib/private/Session/Internal.php
Zeile: 185

#0 [internal function]: OC\Session\Internal->trapError(2, 'session_name():...', '/var/www/nextcl...', 209, Array)
#1 [internal function]: session_name('ocwn0g3teaag')
#2 /var/www/nextcloud/lib/private/Session/Internal.php(209): call_user_func_array('session_name', Array)
#3 /var/www/nextcloud/lib/private/Session/Internal.php(55): OC\Session\Internal->invoke('session_name', Array)
#4 /var/www/nextcloud/lib/base.php(424): OC\Session\Internal->__construct('ocwn0g3teaag')
#5 /var/www/nextcloud/lib/base.php(649): OC::initSession()
#6 /var/www/nextcloud/lib/base.php(1091): OC::init()
#7 /var/www/nextcloud/index.php(35): require_once('/var/www/nextcl...')
#8 {main}

you have to run your code earlier, before any HTTP response is built.

After I restarted Next Clound Pi the situation is still so.
could you tell me how can I fix this ? must I reinstall nextcloud pi ?

I am a bit curiours about Local Development. I do not see any document that we can develop the app on local envinroment. It is only an instruction for setting on Linux or Debian Server. Therefore I do not understand how other programmer can make a new app in Nextcloud.

thanks for your answers