Cannot modify header information

I've noticed increasing number of this in error log:

Code:

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PHP Warning: Cannot modify header information - headers already sent by (output started at /var/www/vhosts/***/httpdocs/system/modules/core/library/Contao/Template.php:303) in /var/www/vhosts/***/httpdocs/system/helper/functions.php on line 130
#0 [internal function]: __error(2, 'Cannot modify h...', '/var/www/vhosts...', 130, Array)
#1 /var/www/vhosts/****/httpdocs/system/helper/functions.php(130): header('HTTP/1.1 500 In...')
#2 /var/www/vhosts/****/httpdocs/system/helper/functions.php(115): show_help_message()
#3 [internal function]: __exception(Object(Exception))
#4 {main}

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What could cause it, and what the possible solution could be?

Thanks.

Contao version 3.5.16