I too saw the negative value, but did not quite understand how it got there. It looks like that has triggered the out of range part of the stack trace.
I am able to set grades using a custom scale in my production Moodle (2.3.3+). In the database, the grade value is "-5." Do you know if you are using the most recent release of OUblog?
SUCCESS! I queried the grade field in mdl_oublog and, sure enough, it was 'unsigned'. So it would not take negative numbers.
The fact it was unsigned is strange. In my other 2.2 installation, the field was not designated as signed or unsigned.
In any case, I changed it to signed
alter table mdl_oublog modify grade int signed;
and now it works!
Thanks, Rob, for talking me though this!
NOT able to get the certificate on quiz restriction to get minimum grades
