There is some issues with the reports. (I haven't had a chance to investigate to a more meaningful info but this should be enough for the team to look into the culprit)

For started the reports redirect to a "not found" error from the MCP
https://gyazo.com/ee8e478017398dbbc7fa6c9c808f994a

there after if you file other reports you would get:

"Return to the index page
General Error
SQL ERROR [ mysqli ]

Duplicate entry '0' for key 'PRIMARY' [1062]

An SQL error occurred while fetching this page. Please contact the Board Administrator if this problem persists."

some more info:

====
General Error
SQL ERROR [ mysqli ]

Duplicate entry '0' for key 'PRIMARY' [1062]

SQL

INSERT INTO phpbb_reports (reason_id, post_id, pm_id, user_id, user_notify, report_closed, report_time, report_text, reported_post_text, reported_post_uid, reported_post_bitfield, reported_post_enable_bbcode, reported_post_enable_smilies, reported_post_enable_magic_url) VALUES (1, 3, 0, 2, 1, 0, 1474495894, '', '<t>awd awd awd adwaw da</t>', '', '3gqhiqtt', '1', '1', '1')

BACKTRACE

FILE: (not given by php)
LINE: (not given by php)
CALL: msg_handler()

FILE: [ROOT]/phpbb/db/driver/driver.php
LINE: 999
CALL: trigger_error()

FILE: [ROOT]/phpbb/db/driver/mysqli.php
LINE: 193
CALL: phpbb\db\driver\driver->sql_error()

FILE: [ROOT]/phpbb/report/report_handler.php
LINE: 100
CALL: phpbb\db\driver\mysqli->sql_query()

FILE: [ROOT]/phpbb/report/report_handler_post.php
LINE: 69
CALL: phpbb\report\report_handler->create_report()

FILE: [ROOT]/phpbb/report/controller/report.php
LINE: 151
CALL: phpbb\report\report_handler_post->add_report()

FILE: (not given by php)
LINE: (not given by php)
CALL: phpbb\report\controller\report->handle()

FILE: [ROOT]/vendor/symfony/http-kernel/HttpKernel.php
LINE: 139
CALL: call_user_func_array()

FILE: [ROOT]/vendor/symfony/http-kernel/HttpKernel.php
LINE: 62
CALL: Symfony\Component\HttpKernel\HttpKernel->handleRaw()

FILE: [ROOT]/app.php
LINE: 35
CALL: Symfony\Component\HttpKernel\HttpKernel->handle()
====================

None of this happens under phpBB 3.1.9