On a V10 (Drupal 9) portal site, log in as superuser or admin user. Enable Database logging i.e. enable the 'dblog' module. Navigate to /admin/config/system/mail_subscribers. This leads to a white screen. Then navigate to /admin/reports/dblog. Choose the last log message which is an error message.
The screenshot below shows the error message.
I applied a code fix to the mail subscribers module then zipped the module and uploaded it to the site. The route /admin/config/system/mail_subscribers then displayed the configuration form without any errors.
I have created an issue and pull request on Github: https://github.com/ibm-apiconnect/devportal/issues/44
Review pull request, merge if good.
Fixed
Code
Thanks for letting us know. The fix has been made and will be released in a couple of weeks in the next release.
FYI just raising the issue on github.com was sufficient, no need to raise duplicate ones here too.