A security fix in 1.6.2 means that you can't have HTML in the mod log action anymore.
How do I disable HTML in mod log?
Well you've got a plugin adding a mod log action with HTML, it's not something you 'disable'. No default mod logs put HTML in the mod log action.
It is a "bug" as the software puts in the HTML. It will be resolved in the next maintenance release, although we won't be able to do much with the existing logs.