So today when I tried to report myself through the console for testing the messages I got a stacktrace that displayed an SQL error.
Stacktrace: http://hastebin.com/heqakemaxu.coffee

I'm running Spigot 1.10 and I downloaded the appropriate version (v5.10.1) from Bukkit.
The stacktrace says that the column 'asignee_id' on table 'bm_player_reports' cannot be null. So ironically I made it null by running the following SQL Query:

ALTER TABLE bm_player_reports
CHANGE assignee_idassignee_id` binary(16) NULL AFTER state_id;

And next time I tried reporting it went trough without a hassle. Fixed it by making the column null.
So my question is: Is this a bug or a confusion with my version and my head?

Most certainly a bug. Someone else reported this a while ago. I've just found what's causing it. Thanks for bringing it up.