It is due repor rid is taken as int and gives an string.
I have same issue and i do some code modos bit reviewing i see the la test file is fine.
Try todo upload report.php from the entire package and let us know if error persist.
Ok i reviewed new and old versions and i think there are some missing code in some template or file, but i did not find yet exactly what file have to been changed, due here is taken as int value the reason, i think we have an old code somewhere on our forums.
I try to download latest 1.8.9 version of MyBB but no way in a few days i tryed and i can not download ever the download file is on 0kb and never download.
I think the problem is on modal loaded we have to set values as id besides reason, because we have spam, others, etc on values and the right values are their ids. 1,2,3,4,etc.
So that is the problem, i solved uploading a modified file till now, i share here if you wanna test but it's recommended to up to date templates and files to know where the problem is really.