Problem arises because of the following

$USER->id is 0, set on line 292 of lib/sessionlib.php (check_user_initialised)

executes last statement of this if statement on line 262 of user/profile.php
if ($currentuser
or $user->maildisplay == 1
or has_capability('moodle/course:useremail', $context)
or ($user->maildisplay == 2 and enrol_sharing_course($user, $USER))) {

which ends up testing !empty($user2->id) on line 268 of lib/enrollib.php (enrol_get_shared_courses)
where $user2 === $USER

The empty function (http://php.net/manual/en/function.empty.php) returns true when the variable is 0. This is not the wanted behaviour.

A fix would be to change this to use isset instead of !empty

A stack trace is below, a patch is attached

Coding error detected, it must be fixed by a programmer: PHP catchable fatal error

More information about this error

Debug info: Object of class stdClass could not be converted to string
Stack trace:
line 359 of /lib/setuplib.php: coding_exception thrown
line ? of unknownfile: call to default_error_handler()
line 636 of /lib/dml/pgsql_native_moodle_database.php: call to pg_query_params()
line 1587 of /lib/dml/moodle_database.php: call to pgsql_native_moodle_database->get_recordset_sql()
line 297 of /lib/enrollib.php: call to moodle_database->record_exists_sql()
line 237 of /lib/enrollib.php: call to enrol_get_shared_courses()
line 267 of /user/profile.php: call to enrol_sharing_course()