List Form entries or short codes found in error_log.txt

Dear support team,

not sure why but I actually find such entries in error log:
Screenshot > https://www.screencast.com/t/GDzYT17cKy0

In example:
[31-May-2018 12:22:14 UTC] Array
(
[email] => [email protected]
[firstname] => tester
[lastname] => test
)

while someone is doing a newsletter subscription i.e. https://www.worldcheckin.com/affiliate/
You can also use https://www.worldcheckin.com/test-forms/ for testings

I am looking forward to your help.

BR, Alex

P.S. Maybe this is also related to inbound forms as I see no further details in error log.
Additional support query made here > https://support.inboundnow.com/support/topic/list-form-entries-or-short-codes-found-in-error_log-txt/