Cannot modify header information – headers already sent by

I tried using your support system and the ticket never submits, so I’m using this forum instead.

We’re having an issue with the Gravity PDF plugin generating the following warnings in the error_log continuously on our site. I’ve disabled the plugin for now because it is interfering with the functionality of our site:

—————————————————————————————

[16-Jan-2019 04:37:35 America/Montreal] PHP Warning: Cannot modify header information – headers already sent by (output started at …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php:365) in …/wp-includes/pluggable.php on line 1219
[16-Jan-2019 07:20:12 America/Montreal] PHP Warning: file_get_contents(/tmp/gravitypdf-0f2ae3894550d5f7b9310dc76ed06e5f/mpdf/tmp_directory_test.php): failed to open stream: No such file or directory in …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php on line 365
[16-Jan-2019 07:20:13 America/Montreal] PHP Warning: session_start(): Cannot send session cache limiter – headers already sent (output started at …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php:365) in …/wp-content/plugins/gravity-forms-duplicate-prevention/gravityforms-duplicateprevention.php on line 74
[16-Jan-2019 07:25:12 America/Montreal] PHP Warning: file_get_contents(/tmp/gravitypdf-0f2ae3894550d5f7b9310dc76ed06e5f/mpdf/tmp_directory_test.php): failed to open stream: No such file or directory in …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php on line 365
[16-Jan-2019 07:25:12 America/Montreal] PHP Warning: session_start(): Cannot send session cookie – headers already sent by (output started at …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php:365) in …/wp-content/plugins/gravity-forms-duplicate-prevention/gravityforms-duplicateprevention.php on line 74
[16-Jan-2019 07:25:12 America/Montreal] PHP Warning: session_start(): Cannot send session cache limiter – headers already sent (output started at …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php:365) in …/wp-content/plugins/gravity-forms-duplicate-prevention/gravityforms-duplicateprevention.php on line 74
[16-Jan-2019 07:25:12 America/Montreal] PHP Warning: Cannot modify header information – headers already sent by (output started at …/wp-content/plugins/gravity-forms-pdf-extended/src/model/Model_Install.php:365) in …/wp-content/themes/ranger-design/inc/configurator.php on line 17
————————————————————————–

How can we diagnose and fix this issue?

Thanks