Plugn causes diverse errors in Moodle 4.1

[paaskynen]

The OU Blog plugin causes errors when activity completion is turned on. In some cases the teacher will see
Exception - Unsupported operand types: int + string
In other cases the error only affects students:
Exception - Call to undefined function mod_oublog\completion\oublog_get_displayname()

What causes these? Thanks in advance for your reply.