[Forum Manager > Delete Forum] BIGINT UNSIGNED value is out of range

[Scandal]

Hello all

Well, I'm troubled into this problem:
We have a lots of forums with subforms (vB3.8.4).
I'm trying to remove a header forum (so also all its subforums will deleted) on Forum Manager > (dropdown menu) Delete Forum

The process is taking enough and suddenly this db error appears:

Database error in vBulletin 3.8.4:

Invalid SQL:

				UPDATE vb3_thread
				SET attach = attach -
				CASE
					 WHEN threadid = 24894 THEN 1
					ELSE 0
				END
				WHERE threadid IN (-1,24894);

MySQL Error   : BIGINT UNSIGNED value is out of range in '(`xxxx`.`vb3_thread`.`attach` - (case when (`xxxx`.`vb3_thread`.`threadid` = 24894) then 1 else 0 end))'
Error Number  : 1690

We're talking about an integrated function of vB, not an addon.
Any idea how to fix this issue?